System and method for electrocardiogram analysis and optimization of cardiopulmonary resuscitation and therapy delivery

ABSTRACT

The system and method provide for electrocardiogram analysis and optimization of patient-customized cardiopulmonary resuscitation and therapy delivery. An external medical device includes a housing and a processor within the housing. The processor can be configured to receive an input signal for a patient receiving chest compressions and to select at least one filter mechanism and to apply the filter mechanism to the signal to at least substantially remove chest compression artifacts from the signal. A real time dynamic analysis of a cardiac rhythm is applied to adjust and integrate CPR prompting of a medical device. Real-time cardiac rhythm quality is facilitated using a rhythm assessment meter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/952,039 entitled “Pause Coordination for CPR Artifact Filtering” andU.S. Provisional Application No. 61/952,074 entitled “Compression RateAssessment for Accurate Comb Filtering”, and further this application isa continuation-in-part of U.S. patent application Ser. No. 14/558,610entitled “System and Method for Electrocardiogram Analysis andOptimization of Cardiopulmonary Resuscitation and Therapy Delivery”,which issued Mar. 15, 2016 as U.S. Pat. No. 9,283,400 and is acontinuation of U.S. patent application Ser. No. 13/836,062 entitled“System and Method for Electrocardiogram Analysis and Optimization ofCardiopulmonary Resuscitation and Therapy Delivery”, which issued Dec.2, 2014 as U.S. Pat. No. 8,903,498 and is a continuation-in-part of U.S.patent application Ser. No. 13/676,593, entitled “Filter Mechanism forRemoving ECG Artifact from Mechanical Chest Compressions,” filed Dec.13, 2012, which issued Jul. 21, 2015 as U.S. Pat. No. 9,084,545 andclaims the benefit of: U.S. Provisional Application No. 61/616,874entitled “Visual Rhythm Assessment Meter,” filed Mar. 28, 2012; U.S.Provisional Application No. 61/616,727, entitled “ECG Frequency Analysisduring CPR,” filed Mar. 28, 2012; U.S. Provisional Application No.61/616,973 entitled “An Analysis during CPR Algorithm Utilizing ShockHistory,” filed Mar. 28, 2012; U.S. Provisional Application No.2061/616,660 entitled “Guiding Therapy with Real-Time VF QualityMeasurement,” filed Mar. 28, 2012; U.S. Provisional Application No.61/616,372 entitled “AED Operation Dependent on Previous AnalysisResults,” filed Mar. 27, 2012; and U.S. Provisional Application No.61/616,847 entitled “Method of Integrating Cardiac Rhythm Analysisduring CPR into an AED Algorithm” filed Mar. 28, 2012, all of which arehereby incorporated by reference herein in their entirety.

FIELD

This invention generally relates to medical devices, such as externaldefibrillators and chest compression devices.

BACKGROUND

In normal operation, the heart pumps blood through the various parts ofthe body in a well-orchestrated fashion. The chambers of the heartcontract and expand in a periodic and coordinated harmony, causing theblood to be pumped regularly. In humans, the right atrium sendsdeoxygenated blood into the right ventricle. The right ventricle pumpsthe blood to the lungs, where the blood becomes oxygenated, and fromwhere it returns to the left atrium. The left atrium pumps theoxygenated blood to the left ventricle. The left ventricle, then, expelsthe blood, forcing it to circulate to the various parts of the body.

The heart chambers pump because of the heart's electrical controlsystem. The sinoatrial (SA) node generates an electrical impulse, whichgenerates further electrical signals. These further signals cause theabove-described contractions of the various chambers in the heart, inthe correct sequence. The electrical pattern created by the sinoatrial(SA) node is called a sinus rhythm.

Sometimes, however, the electrical control system of the heartmalfunctions, which can cause the heart to beat irregularly, or not atall. The cardiac rhythm is then generally called an arrhythmia.Arrhythmias may be caused by electrical activity from locations in theheart other than the SA node. Some types of arrhythmia may result ininadequate blood flow, thus reducing the amount of blood pumped to thevarious parts of the body. Some arrhythmias may even result in a SuddenCardiac Arrest (SCA). In a SCA, the heart fails to pump bloodeffectively, and, if not treated promptly, death can occur. In fact, itis estimated that SCA results in more than 250,000 deaths per year inthe United States alone. Further, a SCA may result from a conditionother than an arrhythmia.

One type of arrhythmia associated with SCA is known as VentricularFibrillation (VF). VF is a type of malfunction where the ventricles makerapid, uncoordinated movements, instead of the normal contractions. Whenthat happens, the heart does not pump enough blood to deliver enoughoxygen to the vital organs. The person's condition will deterioraterapidly and, if not reversed in time, the person will die soon, e.g.within ten minutes.

Ventricular Fibrillation can often be reversed using a life-savingdevice called a defibrillator. A defibrillator, if applied properly, canadminister an electrical shock to the heart. The shock may terminate theVF, thus. giving the heart the opportunity to resume pumping blood. IfVF is not terminated, the shock may be repeated, often at escalatingenergies.

A challenge with defibrillation is that the electrical shock must beadministered very soon after the onset of VF. There is not much time:the survival rate of persons suffering from VF decreases by about 10%for each minute the administration of a defibrillation shock is delayed.After about 10 minutes the rate of survival for SCA victims averagesless than 2%.

The challenge of defibrillating early after the onset of VF is being metin a number of ways. First, for some people who are considered to be ata higher risk of VF or other heart arrhythmias, an ImplantableCardioverter Defibrillator (ICD) can be implanted surgically. An ICD canmonitor the person's heart, and administer an electrical shock asneeded. As such, an ICD reduces the need to have the higher-risk personbe monitored constantly by medical personnel. For individuals who arenot an ICD candidates but still in need of monitoring, a portabledefibrillator that can be worn by the individual at risk can be used.

Regardless, VF can occur unpredictably, even to a person who is notconsidered at risk and has not been monitored. As such, VF can beexperienced by many people who lack the benefit of ICD or wearabletherapy. When VF occurs to a person, every minute counts. During VF, ifthe blood is not flowing to the brain, heart, lungs, and other organs,the person's condition deteriorates rapidly. If resuscitation attemptsare to be successful, blood flow must be restored as quickly aspossible. Cardiopulmonary Resuscitation (CPR) is one method of forcingblood flow in a person experiencing cardiac arrest. AEDs analyze thepatient's electrocardiogram (ECG) to decide whether a patient needs ashock. External defibrillators may also prompt the rescuer to providechest compressions, rescue breathing, and/or shocks based on establishedprotocols.

In some cases, it is recognized that patients benefit greatly from CPRprior to defibrillation. Properly administered CPR provides oxygenatedblood to critical organs of a person in cardiac arrest, therebyminimizing the deterioration that would otherwise occur. For patientswith an extended down-time, survival rates are higher if CPR isadministered prior to defibrillation. CPR is often critical for apatient's survival from sudden cardiac arrest and is the primaryrecommended treatment for some patients with some kinds of non-VFcardiac arrest, such as asystole and pulseless electrical activity(PEA). CPR may be a combination of techniques that include chestcompressions to force blood circulation, and rescue breathing to forcerespiration.

In this race against time for human life, being able to, in real-time,understand the optimal amount, durations, pauses, administrationfrequency of CPR in combination of shock therapy, as well as how toimprove and what to do when the CPR quality is poor, is highlydesirable. Being able to monitor and analyze, and customize the CPR andthe rhythm at the same time and in real-time, determine when to startwith a CPR or a shock first, whether to stop altogether, or continue fora longer than routine/protocol-prescribed period to resuscitatesuccessfully, is highly desirable and highly sought after. However,prior attempts, due to issues largely related to noise artifact, havefailed to providing a system and method capable of successful monitoringand analyzing of rhythms, and other physiological signals andparameters, while performing chest compressions.

Furthermore, to-date, the ECG analysis and evaluation at any given pointhas been held independent of the previous sets of results. Analysisalgorithm depends solely on the signal currently being received from thepatient. Typically, this signal might be an ECG signal, but it may alsoinclude other parameters such as the impedance signal or anaccelerometer signal, etc. Administration of CPR follows a protocol inwhich the number of compressions, pauses for breaths, and the timing ofpauses for analysis have been fixed, and often stand independent of theindividual patient's history and needs.

Fixed treatment CPR/shock therapy protocol and rigid analysis algorithmsare sub-optimal in many situations. The initial rhythm that is presentedwhen the defibrillator is first connected to the patient is a strongpredictor of the course of events for that particular patient. Patientswho present with the initial rhythm of VF or ventricular tachycardia(VT) have an approximately 50% chance of being in VF or VT on asubsequent analysis. However, in the subset of patients with anon-shockable initial rhythm and with a no-shock result for everysubsequent analysis, there is only an approximate 7% chance that theywill be in a shockable rhythm on next analysis. This situation is moreextreme for Automated External Defibrillator cases than in Advanced LifeSupport (ALS) care.

Resuscitation researchers have long known that the prognosis forpatients with an initial rhythm of asystole is dire. Resuscitationcenters are increasingly interested in an approach that tailors the nextstep in the patient care based on first ECG analysis, AED shockdecision. For example, if the first AED decision is “shock advised,” AEDanalysis should be prompted by a device and performed every two minutes.However, if the initial AED decision is “no shock,” the remaining AEDprompts should not be acted upon (i.e. re-analyzed) and CPR should beperformed until ALS care arrives. Whether the initial rhythm isshockable, and whether any subsequent analysis gives a shockable result,are powerful predictors of the patient's condition at any point in time.What is sought is a system and method which can take into accountreal-time continuous analysis, results, and representation of a rhythmanalysis and provide advice as to the next action step or sequence ofsteps. In addition to “shock” or “no-shock” advice, a representation ofa relative shockability recommendation on a scale where “shock” is oneend of the scale and “no-shock” the other end of the scale would enablerescuers to optimize shock therapy and CPR at patient-customizedfrequency and time intervals.

Defibrillator users have a strong desire to be able to analyze an ECGsignal and make a shock/no shock decision while chest compressions arebeing performed. Therefore, what is needed is a system and method whichfilters noise artifact and enables ECG and CPR patient-customizedcollaboration, result analysis and evaluation, and optimization of CPRand therapy delivery.

BRIEF SUMMARY

The present description gives instances of devices, systems, softwareand methods, the use of which may help overcome problems and limitationsof the prior art.

When analyzing a relatively noisy data, such as an Electrocardiogram(ECG) during a Cardiopulmonary Resuscitation (CPR), definitiverecommendations are not always possible or most helpful. In certainembodiments, an external medical device may include a housing, an energystorage module within the housing for storing an electrical charge, anda defibrillation port for guiding via electrodes the stored electricalcharge to a person. The device may also include a processor in thehousing configured to receive a signal from a patient receiving chestcompressions and apply at least one filter to remove from the signalchest compression artifacts resulting from the chest compressions beingdelivered to the patient. An advantage over the prior art is that anexternal medical device in accordance with the disclosed technology canpresent to a user a cleaner signal than would otherwise be provided insituations where a patient is receiving chest compressions. Also, thedevice may determine from chest compression artifacts in the patientsignal a chest compression signature that corresponds to at least oneparticular type of chest compression device.

In one embodiment, a real-time analysis of the quality of a cardiacrhythm, such as a quality of a ventricular fibrillation, is continuouslyand automatically performed during CPR. Based on the analysis, theindication is then presented in real-time, by way of an example, a gaugeor a bar graph, a trend line, or a calorimetric scale. The indicationmay be visual, auditory, tactile, or a combination thereof, etc. The CPRis continued as long as the ventricular fibrillation indicates thiscourse of action. The indication can be presented to a human rescuer orcommunicated to a mechanical device, which then, automatically respondsby adjusting the treatment administered sequence, duration, pauses, etc.An “optimal” amount of CPR is also determined prior to defibrillation.Such approach is patient-tailored and an improvement over performing CPRfor a fixed period of time or until a fixed threshold of certainty isreached with respect to CPR/Therapy delivery intervals. If theventricular fibrillation quality continues to improved, the CPR iscontinued rather than stopped, per existing protocols.

These and other features and advantages of this description will becomemore readily apparent from the following Detailed Description, whichproceeds with reference to the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a scene where an external defibrillator is usedto save the life of a person according to embodiments.

FIG. 2 is a table listing two main types of the external defibrillatorshown in FIG. 1, and who they might be used by.

FIG. 3 is a functional block diagram showing components of an externaldefibrillator, such as the one shown in FIG. 1, which is made accordingto embodiments.

FIG. 4 is a functional block diagram showing components of a patient ECGsignal monitoring system according to embodiments.

FIG. 5 is a graphical illustration of a fast Fourier transform of an ECGsignal of an asystolic patient receiving chest compressions from aconventional mechanical chest compression device.

FIG. 6 is a graphical illustration of a fast Fourier transform of an ECGsignal from an asystolic patient receiving chest compressions from amechanical chest compression device having precise frequency controlaccording to embodiments.

FIG. 7 is a graphical illustration of the frequency response of a combfilter suitable for removing chest compression artifacts from an ECGsignal according to embodiments.

FIG. 8 is a graphical illustration of the frequency response of aninverse comb filter suitable for removing chest compression artifactsfrom an ECG signal according to embodiments.

FIG. 9 is a time diagram of patient ECG data in the form of signals.

FIG. 10A is a time diagram of an ECG signal having QRS complexes and nochest compression artifacts.

FIG. 10B is a time diagram of an ECG signal having QRS complexes andchest compression artifacts with no filtering.

FIG. 10C is a time diagram of an ECG signal having QRS complexes andchest compression artifacts with a filter mechanism applied thereto.

FIG. 11A is a time diagram of an ECG signal having no QRS complexes andno chest compression artifacts.

FIG. 11B is a time diagram of an ECG signal having no QRS complexes andchest compression artifacts with no filtering.

FIG. 11C is a time diagram of an ECG signal having no QRS complexes andchest compression artifacts with a filter mechanism applied thereto.

FIG. 12A is a time diagram of a VF signal having no chest compressionartifacts.

FIG. 12B is a time diagram of a VF signal having chest compressionartifacts with no filtering.

FIG. 12C is a time diagram of a VF signal having chest compressionartifacts with a filter mechanism applied thereto.

FIG. 13 is a flowchart for illustrating methods according toembodiments.

FIG. 14 is a flowchart for illustrating other methods according toembodiments.

FIG. 15 is a flowchart for illustrating other methods according toembodiments.

FIG. 16 is a diagram of frequency spectrum monitored over time.

FIG. 17 is a shock index distribution diagram.

FIG. 18A is shock index probability, interpretation, and relatedrecommended actions table.

FIG. 18B is an example of a rhythm assessment meter.

FIG. 18C is an example of a rhythm assessment meter.

FIG. 19 is a time diagram of a VF probability.

FIG. 20 is a flowchart for illustrating methods according toembodiments.

FIG. 21 is a flowchart for illustrating methods according toembodiments.

FIG. 22 is a flowchart for illustrating methods according toembodiments.

FIG. 23 is a time diagram showing unfiltered and filtered ECG signalsduring CPR with a pause, according to embodiments.

FIG. 24 is a time diagram showing unfiltered and filtered ECG signalsduring CPR with a shock, according to embodiments.

FIG. 25 is an example block diagram of a filter mechanism with aprotection switch according to embodiments.

DETAILED DESCRIPTION

As has been mentioned, the present description is about medical devices,methods of operating such medical devices, and a programmed processor tocontrol such medical devices for removing chest compression artifactsfrom an ECG signal for a patient receiving chest compressions andfurther for analyzing the electrocardiogram for optimization of chestcompression and therapy delivery.

Embodiments are now described in more detail.

FIG. 1 is a diagram of a defibrillation scene. A person 82 is lying ontheir back. Person 82 could be a patient in a hospital, or someone foundunconscious, and then turned to be on their back. Person 82 isexperiencing a condition in their heart 85, which could be, by way of anexample, Ventricular Fibrillation (VF).

A portable external defibrillator 100 has been brought close to person82. The portable external defibrillator can also be a wearable or hybriddefibrillator 82. At least two defibrillation electrodes 104, 108 areusually provided with external defibrillator 100, and are sometimescalled electrodes 104, 108. Electrodes 104, 108 are coupled withexternal defibrillator 100 via respective electrode leads 105, 109. Arescuer (not shown) has attached electrodes 104, 108 to the skin ofperson 82. Defibrillator 100 is administering, via electrodes 104, 108,a brief, strong electric pulse 111 through the body of person 82. Pulse111, also known as a defibrillation shock, goes also through heart 85,in an attempt to restart it, for saving the life of person 82.

Defibrillator 100 can be one of different types, each with differentsets of features and capabilities. The set of capabilities ofdefibrillator 100 is determined by planning who would use it, and whattraining they would be likely to have. Examples are now described.

FIG. 2 is a table listing examples of types of external defibrillators,and who they are primarily intended to be used by. A first type ofdefibrillator 100 is generally called a defibrillator-monitor, becauseit is typically formed as a single unit in combination with a patientmonitor. A defibrillator-monitor is sometimes calledmonitor-defibrillator. A defibrillator-monitor is intended to be used bypersons in the medical professions, such as doctors, nurses, paramedics,emergency medical technicians, etc. Such a defibrillator-monitor isintended to be used in a pre-hospital or hospital scenario.

As a defibrillator, the device can be one of different varieties, oreven versatile enough to be able to switch among different modes thatindividually correspond to the varieties. One variety is that of anautomated defibrillator, which can determine whether a shock is neededand, if so, charge to a predetermined energy level and instruct the userto administer the shock. Another variety is that of a manualdefibrillator, where the user determines the need and controlsadministering the shock.

As a patient monitor, the device has features additional to what isminimally needed for mere operation as a defibrillator. These featurescan be for monitoring physiological indicators of a person in anemergency scenario. These physiological indicators are typicallymonitored as signals. For example, these signals can include a person'sfull ECG (electrocardiogram) signals, or impedance between twoelectrodes. Additionally, these signals can be about the person'stemperature, non-invasive blood pressure (NIBP), arterial oxygensaturation/pulse oximetry (SpO2), the concentration or partial pressureof carbon dioxide in the respiratory gases, which is also known ascapnography, and so on. These signals can be further stored and/ortransmitted as patient data.

A second type of external defibrillator 100 is generally called an AED,which stands for “Automated External Defibrillator”. An AED typicallymakes the shock/no shock determination by itself, automatically. Indeed,it can sense enough physiological conditions of the person 82 via onlythe shown defibrillation electrodes 104, 108 of FIG. 1. In its presentembodiments, an AED can either administer the shock automatically, orinstruct the user to do so, e.g. by pushing a button. Being of a muchsimpler construction, an AED typically costs much less than adefibrillator-monitor. As such, it makes sense for a hospital, forexample, to deploy AEDs at its various floors, in case the moreexpensive defibrillator-monitor is more critically being deployed at anIntensive Care Unit, and so on.

AEDs, however, can also be used by people who are not in the medicalprofession. More particularly, an AED can be used by many professionalfirst responders, such as policemen, firemen, etc. Even a person withonly first-aid training can use one. And AEDs increasingly can supplyinstructions to whoever is using them.

AEDs are thus particularly useful, because it is so critical to respondquickly, when a person suffers from VF. Indeed, the people who willfirst reach the VF sufferer may not be in the medical professions.

Increasing awareness has resulted in AEDs being deployed in public orsemi-public spaces, so that even a member of the public can use one, ifthey have obtained first aid and CPR/AED training on their owninitiative. This way, defibrillation can be administered soon enoughafter the onset of VF, to hopefully be effective in rescuing the person.

There are additional types of external defibrillators, which are notlisted in FIG. 2. For example, a hybrid defibrillator can have aspectsof an AED, and also of a defibrillator-monitor. A usual such aspect isadditional ECG monitoring capability.

FIG. 3 is a diagram showing components of an external defibrillator 300made according to embodiments. These components can be, for example, inexternal defibrillator 100 of FIG. 1. Plus, these components of FIG. 3can be provided in a housing 301, which is also known as casing 301.

External defibrillator 300 is intended for use by a user 380, who wouldbe the rescuer. Defibrillator 300 typically includes a defibrillationport 310, such as a socket in housing 301. Defibrillation port 310includes nodes 314, 318. Defibrillation electrodes 304, 308, which canbe similar to electrodes 104, 108, can be plugged in defibrillation port310, so as to make electrical contact with nodes 314, 318, respectively.It is also possible that electrodes can be connected continuously todefibrillation port 310, etc. Either way, defibrillation port 310 can beused for guiding via electrodes to person 82 an electrical charge thathas been stored in defibrillator 300, as will be seen later in thisdocument.

If defibrillator 300 is actually a defibrillator-monitor, as wasdescribed with reference to FIG. 2, then it will typically also have anECG port 319 in housing 301, for plugging in ECG leads 309. ECG leads309 can help sense an ECG signal, e.g. a 12-lead signal, or from adifferent number of leads. Moreover, a defibrillator-monitor could haveadditional ports (not shown), and an other component 325 structured tofilter the ECG signal, e.g., apply at least one filter to the signal soas to remove chest compression artifacts resulting from chestcompressions being delivered to the person 82.

Defibrillator 300 also includes a measurement circuit 320. Measurementcircuit 320 receives physiological signals from ECG port 319, and alsofrom other ports, if provided. These physiological signals are sensed,and information about them is rendered by circuit 320 as data, or othersignals, etc.

If defibrillator 300 is actually an AED, it may lack ECG port 319.Measurement circuit 320 can obtain physiological signals through nodes314, 318 instead, when defibrillation electrodes 304, 308 are attachedto person 82. In these cases, a person's ECG signal can be sensed as avoltage difference between electrodes 304, 308. Plus, impedance betweenelectrodes 304, 308 can be sensed for detecting, among other things,whether these electrodes 304, 308 have been inadvertently disconnectedfrom the person.

Defibrillator 300 also includes a processor 330. Processor 330 may beimplemented in any number of ways. Such ways include, by way of exampleand not of limitation, digital and/or analog processors such asmicroprocessors and digital-signal processors (DSPs); controllers suchas microcontrollers; software running in a machine; programmablecircuits such as Field Programmable Gate Arrays (FPGAs),Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices(PLDs), Application Specific Integrated Circuits (ASICs), anycombination of one or more of these, and so on.

Processor 330 can be considered to have a number of modules. One suchmodule can be a detection module 332, which senses outputs ofmeasurement circuit 320. Detection module 332 can include a VF detector.Thus, the person's sensed ECG can be used to determine whether theperson is experiencing VF.

Another such module in processor 330 can be an advice module 334, whicharrives at advice based on outputs of detection module 332. Advicemodule 334 can include a Shock Advisory Algorithm, implement decisionrules, and so on. The advice can be to shock, to not shock, toadminister other forms of therapy, and so on. If the advice is to shock,some external defibrillator embodiments merely report that to the user,and prompt them to do it. Other embodiments further execute the advice,by administering the shock. If the advice is to administer CPR,defibrillator 300 may further issue prompts for it, and so on.

Processor 330 can include additional modules, such as module 336, forother functions. In addition, if other component 325 is indeed provided,it may be operated in part by processor 330, etc.

Defibrillator 300 optionally further includes a memory 338, which canwork together with processor 330. Memory 338 may be implemented in anynumber of ways. Such ways include, by way of example and not oflimitation, nonvolatile memories (NVM), read-only memories (ROM), randomaccess memories (RAM), any combination of these, and so on. Memory 338,if provided, can include programs for processor 330, and so on. Theprograms can be operational for the inherent needs of processor 330, andcan also include protocols and ways that decisions can be made by advicemodule 334. In addition, memory 338 can store prompts for user 380, etc.Moreover, memory 338 can store patient data.

Defibrillator 300 may also include a power source 340. To enableportability of defibrillator 300, power source 340 typically includes abattery. Such a battery is typically implemented as a battery pack,which can be rechargeable or not. Sometimes, a combination is used, ofrechargeable and non-rechargeable battery packs. Other embodiments ofpower source 340 can include AC power override, for where AC power willbe available, and so on. In some embodiments, power source 340 iscontrolled by processor 330.

Defibrillator 300 additionally includes an energy storage module 350.Module 350 is where some electrical energy is stored, when preparing itfor sudden discharge to administer a shock. Module 350 can be chargedfrom power source 340 to the right amount of energy, as controlled byprocessor 330. In typical implementations, module 350 includes one ormore capacitors 352, and so on.

Defibrillator 300 moreover includes a discharge circuit 355. Circuit 355can be controlled to permit the energy stored in module 350 to bedischarged to nodes 314, 318, and thus also to defibrillation electrodes304, 308. Circuit 355 can include one or more switches 357. Those can bemade in a number of ways, such as by an H-bridge, and so on.

Defibrillator 300 further includes a user interface 370 for user 380.User interface 370 can be made in any number of ways. For example,interface 370 may include a screen, to display what is detected andmeasured, provide visual feedback to the rescuer for their resuscitationattempts, and so on. Interface 370 may also include a speaker, to issuevoice prompts, etc. Interface 370 may additionally include variouscontrols, such as pushbuttons, keyboards, and so on. In addition,discharge circuit 355 can be controlled by processor 330, or directly byuser 380 via user interface 370, and so on.

Defibrillator 300 can optionally include other components. For example,a communication module 390 may be provided for communicating with othermachines. Such communication can be performed wirelessly, or via wire,or by infrared communication, and so on. This way, data can becommunicated, such as patient data, incident information, therapyattempted, CPR performance, and so on.

A feature of a defibrillator can be CPR-prompting. Prompts are issued tothe user, visual or by sound, so that the user can administer CPR.Examples are taught in U.S. Pat. No. 6,334,070 and No. 6,356,785.

System for Filter Mechanism for Removing EGG Artifact from CPR

FIG. 4 is a functional block diagram showing components of a patient ECGsignal monitoring system according to embodiments. The system includesan external medical device 400, such as an external defibrillator,having a housing 401, a display 470 in connection with the housing 401,and a processor 430 within the housing 401. One having ordinary skill inthe art will recognize that systems according to embodiments generallyrequire no additional sensors or sensor mechanisms than those alreadyprovided.

In the example, the system also includes a mechanical chest compressiondevice 485. The mechanical chest compression device 485 may delivercompressions at 100+/−0.01 compressions/minute, which is 1⅔+/−0.00017Hz. Such precise frequency control is unusual for typical chestcompression devices. An ECG signal may thus be corrupted by chestcompression artifacts corresponding to chest compressions delivered bythe chest compression device 485 to the patient 482. Such artifacts mayhave an artifact fundamental frequency of 1⅔ Hz, and the artifact signalmay also contain harmonics of 1⅔ Hz, which will show up at multiples of1⅔ Hz, e.g., 3⅓ Hz, 5.0 Hz, and 6⅔ Hz. The spectral content of thesefrequency components is generally extremely narrow.

The processor 430 may be configured to receive an input signalcontaining ECG data for a patient 482 receiving chest compressions fromthe mechanical chest compression device 485. The input signal may bereceived via an ECG port 419 in connection with the housing 401. Incertain embodiments, the processor 430 is further configured to detectthe chest compressions being delivered to the patient 482.

The processor 430 may be further configured to select at least onefilter mechanism 425, the mechanical chest compression device 485 havinga chest compression frequency f. The mechanical chest compression device485 may provide an indication of the frequency f to the processor 430.

In certain embodiments, the at least one filter mechanism 425 comprisesa comb filter. The comb filter may be non-adaptive. In otherembodiments, the at least one filter mechanism 425 comprises a pluralityof notch filters. Each of the notch filters may be non-adaptive. Onehaving ordinary skill in the art will readily recognize that variousother filter mechanisms may be used in addition to or in place of a combfilter or notch filters.

Certain conventional CPR artifact filters may be adaptive in nature. Asused herein, an adaptive filter generally refers to a filter whosetransfer function is dependent on the input signal. An adaptive filtermay adjust its filter coefficients, center frequency, rolloff, notchwidth, Q, or other characteristic based on the input signal.Non-adaptive filters according to embodiments generally usepredetermined coefficients that may precisely set the transfer functionindependent of the input signal.

It is possible that a device incorporating this invention may includemultiple non-adaptive filters. The appropriate filter may be selectedbased on input signal characteristics, such as the frequency content ofthe ECG signal or impedance signal. Alternatively, the appropriatefilter may be selected by communication with the mechanical chestcompression device, or through a user input selection.

In certain embodiments, the selecting of the at least one filtermechanism 425 is performed responsive to an identification of themechanical chest compression device 485 being used to deliver the chestcompressions to the patient 482. Alternatively or in addition thereto,the processor 430 may be configured to select the at least one filtermechanism 425 responsive to input received from the mechanical chestcompression device 485 delivering the chest compressions to the patient482. In certain embodiments, the processor 430 may be configured toselect the at least one filter mechanism 425 responsive to inputreceived from a user 480.

The processor 430 may be configured to apply the at least one filtermechanism 425 to the ECG data to at least substantially remove chestcompression artifacts from the ECG data, wherein the chest compressionartifacts correspond to the chest compressions being delivered to thepatient 482 by the mechanical chest compression device 485, and whereinthe at least one filter mechanism 425 substantially rejects content inthe frequency f plus content in at least one more frequency that is ahigher harmonic to the frequency f. In certain embodiments, applicationof the at least one filter mechanism 425 to the ECG data reduces anamplitude of the chest compression artifacts by at least 20 dB relativeto the input signal.

The processor 430 may be further configured to cause the display 470 tovisually present the filtered ECG data to the user 480. Alternatively orin addition thereto, the processor 430 may be configured to cause anoptional printer 439 to print out the filtered ECG data. In certainembodiments, the processor 430 may cause the filtered ECG to be stored,e.g., by a memory 438, for later review or downloading to a post-eventreview tool.

In certain embodiments, the processor 430 is preconfigured to apply theat least one filter mechanism 425. In other embodiments, the processor430 may be configured to apply the at least one filter mechanism 425 tothe ECG data responsive to input received from the user 480.

In certain embodiments, the ECG data is received in real-time. In otherembodiments, the ECG data is received in a post-event review. In theseembodiments, the ECG data may have been recorded from defibrillationpatches or an ECG monitor having multiple leads, e.g., three or moreleads. The at least one filter mechanism 425 may be applied to the ECGdata regardless of whether the device that recorded the signal even hadthe at least one filter mechanism 425. Indeed, the ECG data could beprovided, e.g., sent via e-mail, to another user who causes the at leastone filter mechanism 425 to be applied thereto. Post-event filtering maybe used for establishing the time of re-fibrillation or examining thesignal characteristics prior to fibrillation, for example.

For a patient experiencing VF, VF quality measures such as median VFfrequency, Amplitude Spectral Area (AMSA), and the scaling exponent maybe used for deciding when to apply chest compressions to the patient 482and when to defibrillate the patient 482. By applying the at least onefilter mechanism 425, these parameters may be accurately measured duringCPR.

The processor 430 may be configured to determine a pattern of the chestcompression artifacts corresponding to the chest compressions beingdelivered to the patient 482. The pattern may be based on starting andstopping of the chest compressions being delivered to the patient 482,for example. The processor 430 may be configured to determine whether achest compression artifact pattern matches an existing chest compressionsignature. In certain embodiments, the processor 430 may be furtherconfigured to merge information corresponding to the pattern withinformation corresponding to the predetermined pattern responsive to adetermination that the pattern matches the existing chest compressionsignature. In other embodiments, the processor 430 may be configured togenerate a new chest compression signature responsive to a determinationthat the pattern does not match the existing chest compressionsignature.

In certain embodiments, the processor 430 is configured to suppressapplication of the at least one filter mechanism 425 to the ECG dataresponsive to a determination that the mechanical chest compressiondevice 485 is no longer delivering chest compressions to the patient482. The processor 430 may be further configured to resume applicationof the at least one filter mechanism 425 to the ECG data responsive to adetermination that the mechanical chest compression device 485 hasresumed delivery of chest compressions to the patient 482. The presenceand/or absence of chest compressions may be detected using a measurementof the impedance signal. For example, the RMS value of a one-secondwindow of the impedance signal is generally a reliable indicator ofchest compressions.

In certain embodiments, the processor 430 is configured to generate areport, e.g., CPR statistics, corresponding to the chest compressionsthat were delivered to the patient 482. Alternatively or in additionthereto, the processor 430 may be configured to generate a reportcorresponding to the mechanical chest compression device 485 that wasused to deliver the chest compressions to the patient 482.

In certain embodiments, the processor 430 is further configured tomonitor an impedance signal corresponding to the patient. An impedancewaveform could be filtered to remove compression artifacts, for example,to allow for detection of ventilation artifacts or the presence ofcardiac output. The processor 430 may be further configured to detectreturn of spontaneous circulation (ROSC) by applying a signal-averagingfilter to the impedance signal, e.g., combining a comb filter with thesignal-averaging filter.

In certain embodiments, the processor 430 is further configured toanalyze the filtered ECG data. In these embodiments, the processor 430may be further configured to determine a shock/no shock decision basedon the analysis of the filtered ECG data.

In certain embodiments, the chest compressions are manually delivered tothe patient 482 by the rescuer 480. In these embodiments, the rescuer480 may use a metronome while delivering the chest compressions to thepatient 482 in order to deliver compressions at a very precise rate, forexample. The processor 430 may be configured to select the at least onefilter mechanism 425 based at least in part on a chest compression ratecorresponding to the chest compressions being delivered to the patient482. These embodiments may further include informing the rescuer 480whether the CPR is currently effective, i.e., the chest compressions arebeing administered at the correct rate. The rescuer 480 may thus judgewhether to trust the filtered display 470.

In certain embodiments, the device 400 further includes an energystorage module within the housing 401 for storing an electrical chargeand a defibrillation port for guiding via electrodes the storedelectrical charge to the patient 482.

FIG. 5 is a graphical illustration of a fast Fourier transform of an ECGsignal of an asystolic patient, such as patient 482 of FIG. 4, receivingchest compressions from a conventional mechanical chest compressiondevice. As can be seen from the illustrated example, the ECG signal froman asystolic patient generally contains only artifacts because thepatient has no active cardiac signal. Multiple spectral peaks areevident, with the fundamental frequency of the chest compressionsappearing at 1.6 Hz and other peaks representing harmonic frequencies.The width of these spectral peaks varies from approximately 0.15 Hz atthe fundamental frequency up to approximately 0.5 Hz for the 6^(th)harmonic (10 Hz). It would be difficult to remove the CPR artifact fromthe illustrated signal due to the requirement for a relatively widefilter, which would necessarily remove much of the cardiac signal, thuscausing distortion that would adversely impact the signal.

Signals corresponding to conventional mechanical CPR devices generallyhave only broad spectral peaks, and the locations of such peaks aretypically not precisely controlled. The fundamental frequency may varyfrom one device to another, or from one application to another. Forexample, the fundamental frequency may vary from 1.4 Hz to 1.7 Hz. Suchvariation generally prevents application of a non-adaptive filter, e.g.,a comb filter, with a narrow stop band.

Conventional CPR artifact filters have been unsuccessful at removing CPRartifacts, in part, because they typically focus on removing thefundamental frequency while paying little, if any, attention to theharmonic frequencies. In the example illustrated by FIG. 5, the 12^(th)harmonic is only about 11 dB down from the fundamental frequency. In oneexample, to produce a clean ECG signal, CPR artifacts usually need to beattenuated by at least 20 dB, and possibly as much as 40 dB. In order toclean up the signal, frequencies up to at least the 12^(th) harmonicmust typically be removed.

FIG. 6 is one embodiment of a graphical illustration of a fast Fouriertransform of an ECG signal from an asystolic patient, such as thepatient 482 of FIG. 4, receiving chest compressions from a mechanicalchest compression device having precise frequency control according toembodiments. The spectral peaks of the artifacts generated by thisdevice are typically very narrow, e.g., less than 0.1 Hz wide. Thisnarrow spectral content enables the cardiac ECG signal to be separatedfrom chest compression artifact. As with the signal of FIG. 5, multiplefrequency harmonics are present in the signal of FIG. 6, in which the5^(th) harmonic is less than 20 dB down and the 11^(th) harmonic is lessthan 40 dB down. In order to clean up the signal, harmonics up to atleast the 5^(th) harmonic, and possibly as high as the 11^(th) harmonic,should be removed.

FIG. 7 is a graphical illustration of one embodiment of the frequencyresponse of a comb filter, such as a high-Q comb filter, e.g., Q=16,suitable for removing chest compression artifacts from an ECG signalaccording to embodiments. A comb filter intrinsically removes thefundamental frequency and all of the harmonics. If the Q is setrelatively high, e.g. 16, the filter will surgically remove the artifactfrequencies and leave the other frequencies relatively untouched.

In general, high-Q filters are more frequency-selective than low-Qfilters. For example, a comb filter having Q=16 will generally have a 3dB notch width of about 0.1 Hz, whereas a comb filter having Q=4 willtypically have a 3 dB notch width of about 0.5 Hz. A filter having Q=2has approximately a 3 dB notch width of about 1 Hz and usually removesalmost as much of the signal as it retains. A lower-Q filter willgenerally remove more artifacts from a signal than a high-Q filter butwill also remove more of the signal itself. In addition, a low-Q filtertends to produce more ringing, which often provides additionaldistortion.

In one embodiment, to effectively remove CPR artifacts resulting fromapplication of a conventional chest compression device, a very low-Qfilter is preferable. Assuming that at least 20 dB of attenuation isneeded, even a filter having Q=2 may not be effective in removing theartifact from the signal due to the spectral peaks of the artifact beingtoo tall and too broad.

Because the spectral content of a mechanical CPR device according toembodiments is generally narrow, a high-Q filter is used to remove thecompression artifact and retain the cardiac ECG signal with littledistortion. Because a mechanical CPR device according to embodimentsgenerally produces compressions at a precisely known frequency, theartifact may be filtered using a non-adaptive filter. Combining thesetwo aspects (narrow frequency content and precise frequency control)according to embodiments thus enabled a high-Q comb filter to be used asan effective filter for removing CPR artifacts from the input signal.

By way of an example, the following is a Z transform of a suitable combfilter:

${H(z)} = \frac{a\left( {z^{- 1} - z^{- n}} \right)}{1 - {bz}^{- n}}$where “a” is a gain constant, “b” sets the filter Q, and “n” is aninteger that sets the notch frequencies. The Q of this filter may be setby a single coefficient, the gain constant “b.” For example, b=0.82 fora Q of 16. The value of “n” and the sample frequency may be set tolocations of the comb notch frequencies. In situations where n=75 andthe sample rate is 125 Hz, for example, the notch frequencies would be1⅔ Hz, 3⅓ Hz, 5.0 Hz, etc.

In a further embodiment, a comb filter generally introduces very littlesignal delay. The signal is typically delayed only one sample, which is8 milliseconds at 125 Hz, for example. From a user's standpoint, thisdelay is imperceptible. This is in contrast to certain filterstructures, such as finite impulse response (FIR) filters, that candelay the signal by a second or more. Such delay could lead to amisalignment between the filtered ECG and other signals, such as theunfiltered ECG or an invasive blood pressure waveform, confusing to theuser. Alternatively or in addition thereto, a collection of narrow notchfilters, e.g., one filter for the fundamental frequency and one forevery harmonic that needs to be removed, may be used. This small delaymay make a comb filter particularly suitable for an ECG display, inwhich signal delays or misalignment with other monitoring parameters maybe objectionable.

FIG. 8 is a graphical illustration of the frequency response of aninverse comb filter suitable for detecting chest compression artifactsfrom an ECG signal according to embodiments. An inverse comb filter isgenerally similar to a comb filter except that it passes the combfrequencies instead of rejecting them. Such an inverse comb filter maybe particularly suitable for detection of mechanical compressionsdelivered at certain rates, e.g., 100 compressions/minute.

FIG. 9 is a time diagram of patient ECG data in the form of signals. TheECG data in this example is presently exhibiting an impulsive waveformhaving signal spikes or peaks that include both positive peaks andnegative peaks. For example, the ECG data of FIG. 9 may generallycorrespond to a patient, such as the patient 482 of FIG. 4, that isneither experiencing a cardiac event nor receiving chest compressions,e.g., from a chest compression device such as the mechanical chestcompression device 485 of FIG. 4.

FIG. 10A is a time diagram of an ECG signal having QRS complexes and nochest compression artifacts. The QRS complexes generally include bothpositive peaks and negative peaks. As with the ECG data of FIG. 9, theECG signal of FIG. 10A may generally correspond to a patient, such asthe patient 482 of FIG. 4, that is neither experiencing a cardiac eventnor receiving chest compressions, e.g., from a chest compression devicesuch as the mechanical chest compression device 485 of FIG. 4.

FIG. 10B is a time diagram of an ECG signal having QRS complexes andchest compression artifacts with no filtering. For example, the ECGsignal of FIG. 10B may generally correspond to a patient, such as thepatient 482 of FIG. 4, that is not necessarily experiencing a cardiacevent but is presently receiving chest compressions, e.g., from a chestcompression device such as the mechanical chest compression device 485of FIG. 4. As can be readily ascertained by even a causal viewer, theQRS complexes in the ECG signal are at least partially, if not fully,obscured by the chest compression artifacts.

FIG. 10C is a time diagram of an ECG signal having QRS complexes andchest compression artifacts with a filter mechanism, such as the filtermechanism 425 of FIG. 4, applied thereto. The effect of such applicationis readily apparent. Indeed, the time diagram of FIG. 10C issignificantly closer in appearance to the time diagram of FIG. 10A thanto the time diagram of FIG. 10B. One can even readily discern P-wavesand inverted T-waves in the time diagram. Further, a QRS detector coulduse the filtered waveform to provide an accurate intrinsic heart rateindication during delivery of chest compressions to the patient.

FIG. 11A is a time diagram of an ECG signal having no QRS complexes andno chest compression artifacts. For example, the ECG signal of FIG. 11Amay generally correspond to a patient, such as the patient 482 of FIG.4, that is currently experiencing asystole but to whom chestcompressions have not yet been applied, e.g., from a chest compressiondevice such as the mechanical chest compression device 485 of FIG. 4.

FIG. 11B is a time diagram of an ECG signal having no QRS complexes andchest compression artifacts with no filtering. For example, the ECGsignal of FIG. 11B may generally correspond to a patient, such as thepatient 482 of FIG. 4, that is currently experiencing asystole and towhom chest compressions are being concurrently applied, e.g., from achest compression device such as the mechanical chest compression device485 of FIG. 4.

FIG. 11C is a time diagram of an ECG signal having no QRS complexes andchest compression artifacts with a filter mechanism, such as the filtermechanism 425 of FIG. 4, applied thereto. As with the time diagram ofFIG. 10C, the effect of such application is readily apparent here.Indeed, the time diagram of FIG. 10C is significantly closer inappearance to the time diagram of FIG. 10A than to the time diagram ofFIG. 10B.

FIG. 12A is a time diagram of a VF signal having no chest compressionartifacts. For example, the VF signal may generally correspond to apatient, such as the patient 482 of FIG. 4, that is currentlyexperiencing VF but to whom chest compressions have not yet beenapplied, e.g., from a chest compression device such as the mechanicalchest compression device 485 of FIG. 4.

FIG. 12B is a time diagram of a VF signal having chest compressionartifacts with no filtering. For example, the VF signal may generallycorrespond to a patient, such as the patient 482 of FIG. 4, that iscurrently experiencing VF and to whom chest compressions are beingconcurrently applied, e.g., from a chest compression device such as themechanical chest compression device 485 of FIG. 4.

FIG. 12C is a time diagram of a VF signal having chest compressionartifacts with a filter mechanism, such as the filter mechanism 425 ofFIG. 4, applied thereto. As with the time diagrams of FIGS. 10C and 11C,the effect of such application is readily apparent here. Indeed, thesignal presented by the time diagram of FIG. 10C is significantly closerin appearance to the signal presented by the time diagram of FIG. 10Athan to the signal presented by the time diagram of FIG. 10B.

The functions of this description may be implemented by one or moredevices that include logic circuitry. The device performs functionsand/or methods as are described in this document. The logic circuitrymay include a processor that may be programmable for a general purpose,or dedicated, such as microcontroller, a microprocessor, a DigitalSignal Processor (DSP), etc. For example, the device may be a digitalcomputer like device, such as a general-purpose computer selectivelyactivated or reconfigured by a computer program stored in the computer.Alternately, the device may be implemented by an Application SpecificIntegrated Circuit (ASIC), etc.

Moreover, methods are described below. The methods and algorithmspresented herein are not necessarily inherently associated with anyparticular computer or other apparatus. Rather, various general-purposemachines, computers may be used with programs in accordance with theteachings herein, or it may prove more convenient to construct morespecialized apparatus to perform the required method steps. The requiredstructure for a variety of these machines will become apparent from thisdescription.

In all cases there should be borne in mind the distinction betweenmethods in this description, and the method of operating a computingmachine. This description relates both to methods in general, and alsoto steps for operating a computer and for processing electrical or otherphysical signals to generate other desired physical signals.

Programs are additionally included in this description, as are methodsof operation of the programs. A program is generally defined as a groupof steps leading to a desired result, due to their nature and theirsequence. A program is usually advantageously implemented as a programfor a computing machine, such as a general-purpose computer, a specialpurpose computer, a microprocessor, etc.

Storage media are additionally included in this description. Such media,individually or in combination with others, have stored thereoninstructions of a program made according to the invention. A storagemedium according to the invention is a computer-readable medium, such asa memory, and is read by the computing machine mentioned above.

Performing the steps or instructions of a program requires physicaldevice, physical manipulations of physical quantities. Usually, thoughnot necessarily, these quantities may be transferred, combined,compared, and otherwise manipulated or processed according to theinstructions, and they may also be stored in a computer-readable medium.These quantities include, for example electrical, magnetic, andelectromagnetic signals, and also states of matter that can be queriedby such signals. It is convenient at times, principally for reasons ofcommon usage, to refer to these quantities as bits, data bits, samples,values, symbols, characters, images, terms, numbers, or the like. Itshould be borne in mind, however, that all of these and similar termsare associated with the appropriate physical quantities, and that theseterms are merely convenient labels applied to these physical quantities,individually or in groups.

This detailed description is presented largely in terms of flowcharts,display images, algorithms, and symbolic representations of operationsof data bits within at least one computer readable medium, such as amemory. Indeed, such descriptions and representations are the type ofconvenient labels used by those skilled in programming and/or the dataprocessing arts to effectively convey the substance of their work toothers skilled in the art. A person skilled in the art of programmingmay use these descriptions to readily generate specific instructions forimplementing a program according to the present invention.

Often, for the sake of convenience only, it is preferred to implementand describe a program as various interconnected distinct softwaremodules or features or algorithm, individually and collectively alsoknown as software. This is not necessary, however, and there may becases where modules are equivalently aggregated into a single programwith unclear boundaries. In any event, the software modules or featuresof this description may be implemented by themselves, or in combinationwith others. Even though it is said that the program may be stored in acomputer-readable medium, it should be clear to a person skilled in theart that it need not be a single memory, or even a single machine.Various portions, modules or features of it may reside in separatememories, or even separate machines. The separate machines may beconnected directly, or through a network, such as a local access network(LAN), or a global network, such as the Internet.

It will be appreciated that some of these methods may include softwaresteps that may be performed by different modules of an overall softwarearchitecture. For example, data forwarding in a router may be performedin a data plane, which consults a local routing table. Collection ofperformance data may also be performed in a data plane. The performancedata may be processed in a control plane, which accordingly may updatethe local routing table, in addition to neighboring ones. A personskilled in the art will discern which step is best performed in whichplane.

An economy is achieved in the present document in that a single set offlowcharts is used to describe both programs, and also methods. So,while flowcharts are described in terms of boxes, they can mean bothmethod and programs.

For this description, the methods may be implemented by machineoperations. In other words, embodiments of programs are made such thatthey perform methods of the invention that are described in thisdocument. These may be optionally performed in conjunction with one ormore human operators performing some, but not all of them. As per theabove, the users need not be collocated with each other, but each onlywith a machine that houses a portion of the program. Alternately, someof these machines may operate automatically, without users and/orindependently from each other.

Filter Mechanism Method for Removing EGG Artifact from CPR

FIG. 13 is a flowchart 1300 for illustrating methods according toembodiments. The methods of flowchart 1300 may be practiced by systems,devices, and software according to embodiments. For example, the methodsillustrated by flowchart 1300 can be performed by the external medicaldevice 400 illustrated in FIG. 4.

According to an operation at 1302, a signal containing ECG data for apatient receiving chest compressions from a mechanical chest compressiondevice as the mechanical chest compression device 485 of FIG. 4 isreceived. The mechanical chest compression device has a chestcompression frequency f. Certain embodiments may include detecting thechest compressions being delivered to the patient.

According to an optional operation at 1304, the mechanical chestcompression device is identified. In certain embodiments, a processor,such as the processor 430 of FIG. 4, may perform the identifying. Inother embodiments, the chest compression device may send identifyinginformation to the processor.

According to a next operation at 1306, at least one filter mechanism isselected. The selecting may be based on a chest compression rate, asample rate of the ECG data, an identification of the mechanical chestcompression device being used to deliver the chest compressions to thepatient, or a combination thereof.

The at least one filter mechanism may include a comb filter, an inversecomb filter, a matched filter, a plurality of notch filters, or anysuitable combination thereof. In embodiments including a comb filter,the comb filter may be non-adaptive. In embodiments including aplurality of notch filters, each of the notch filters may benon-adaptive.

According to a next operation at 1308, the at least one filter mechanismselected at 1306 is applied to the received signal to at leastsubstantially remove chest compression artifacts from the ECG data,wherein the chest compression artifacts correspond to the chestcompressions being delivered to the patient by the mechanical chestcompression device.

According to a next operation at 1310, the filtered ECG data may bevisually presented to a user, e.g., via a display such as the display470 illustrated in FIG. 4.

According to an optional operation at 1312, the filtered ECG data isanalyzed. Any of a wide variety of suitable techniques may be used inthe analyzing.

According to an optional operation at 1314, a shock/no shock decision isdetermined based on the analyzing. For example, a shock decision may bedetermined based on a result of the analyzing that indicates no QRScomplexes are present in the filtered ECG data. Conversely, a no shockdecision may be determined based on a result of the analyzing thatindicates QRS complexes are present in the filtered ECG data.

In certain embodiments, methods may further include storing anelectrical charge and guiding via electrodes the stored electricalcharge to the patient.

FIG. 14 is a flowchart 1400 for illustrating other methods according toembodiments. In particular, the flowchart 1400 corresponds to theoperation 1308 of the methods illustrated by the flowchart 1300 of FIG.13.

According to an operation at 1402, content in the frequency f issubstantially rejected by the at least one filter mechanism.Consequently, an amplitude of chest compression artifacts at thefrequency f may be reduced, e.g., by at least 20 dB relative to theinput signal.

According to a next operation at 1404, content in at least one morefrequency that is a higher harmonic to the frequency f is substantiallyrejected by the at least one filter mechanism. As with the content inthe frequency f, an amplitude of chest compression artifacts, at eachhigher harmonic to the frequency f may be reduced, e.g., by at least 20dB relative to the input signal.

FIG. 15 is a flowchart for illustrating other methods according toembodiments.

According to an operation at 1502, chest compression artifacts in asignal containing ECG data for a patient receiving chest compressionsfrom a mechanical chest compression device are evaluated. For example, apattern of chest compression artifacts corresponding to the chestcompressions being delivered to the patient may be determined. Thepattern may be based on starting and stopping of the chest compressionsbeing delivered to the patient, for example.

According to an operation at 1504, a determination is made as to whetherthe pattern matches an existing chest compression signature. Responsiveto a determination that the pattern matches an existing chestcompression signature, the method proceeds to an operation at 1506;otherwise, the method proceeds to an operation at 1508.

According to the operation at 1506, a filter mechanism, such as thefilter mechanism 425 of FIG. 4, is selected based on the existing chestcompression signature. In certain embodiments, information correspondingto the pattern may be merged with information corresponding to thepredetermined pattern.

According to the operation at 1508, a new chest compression signature isgenerated based on the pattern.

According to a next operation at 1510, a filter mechanism, such as thefilter mechanism 425 of FIG. 4, is selected based on the chestcompression signature generated at 1508.

Certain embodiments may include determining whether the mechanical chestcompression device is still delivering chest compressions to thepatient. These embodiments may further include suppressing the applyingresponsive to a determination that the mechanical chest compressiondevice is no longer delivering chest compressions to the patient.

Certain embodiments may include monitoring an impedance signalcorresponding to the patient. These embodiments may further includeapplying a signal-averaging filter to the impedance signal to detect areturn of spontaneous circulation (ROSC).

ECG Frequency Analysis During CPR

Additionally, in some cases, patients with more than five minutes of VFbenefit from one to two minutes of CPR prior to defibrillation. VFfrequency decreases with VF duration. CPR increases the VF frequency.The VF frequency may act as a surrogate for the condition of thecardiomyocytes, which deteriorate when deprived of circulation (e.g.during unsupported VF) and “perk up” when oxygenated (during CPR).Patient outcomes are better if a shock is delivered with the heart cellsare more “alert.” Shocks delivered to poor quality VF are likely toresult in asystole, PEA, or more VF. Although some VF patients clearlybenefit from CPR prior to defibrillation, the optimal amount of CPR has,to-date, proven difficult to assess.

The present subject matter discloses a system and method foradministering an optimal amount of CPR to a patient such as the patient482 of FIG. 4, by allowing the amount of CPR to be adjusted tocompensate for variable downtimes and for variable CPR quality,especially in cases when a rescuer performing CPR is a person.

FIG. 16 demonstrates an embodiment representation of ECG frequencyspectrum monitored over time, where frequency is measured in Hertz (Hz)and time in seconds (sec). In this example, the optimal amount of CPR isdetermined prior to defibrillation and coordination of CPR anddefibrillation based on the ECG analysis. VF at about 120 seconds iscontinuing unsupported until about 410 seconds. At that point, CPR isstarted. CPR continues until about 830 seconds, at which point a seriesof defibrillation shocks are provided. FIG. 16 demonstrates that thepeak VF frequency started at about 10 Hz (at about 120 seconds) andthen, began to drop and spread out. After CPR created artifact but theVF frequency can be seen to increase for 200-300 seconds, after which itbegan to gradually fall again.

The peak VF frequency indicates the time when the heart is able tosupport circulation. The improvement during CPR occurs as oxygenatedblood is circulated to the heart. The deterioration that occurs near theend of the CPR period occurs because the amount of circulation providedduring CPR is inadequate to support the metabolic needs of the heart.

FIG. 16 further illustrates frequency spectrum spreading out over timeduring unsupported VF. At 120 seconds the distribution of frequencies isonly a few Hertz, but at 400 seconds it is approximately 8-10 Hz. AfterCPR starts, the VF frequency increases and the distribution offrequencies is also reduced. By 750 seconds the frequency distributionis down to 2-3 Hz. Both VF frequency and VF frequency range provide anindication of VF quality. Determination in how much CPR to performbefore defibrillating is administered is based on the VF quality. Asdemonstrated by FIG. 16, the optimal time to shock is either the pointwhen the VF frequency is at a maximum or when the spread of frequenciesis at a minimum. In a further embodiment, the above-described measuresare combined with each other or with other measures such as VF amplitudeto provide a stronger indication VF quality. For example, calculatingthe area under the frequency curve provides a single measure, whichincludes both frequency and distribution.

FIG. 16 also illustrates an approach of effectively compensating fordiffering down times and VF quality. Poor CPR quality results in littleor no increase in VF quality, or retards the speed at which an optimalvalue is reached. A shorter (or longer) VF duration may cause the heartto respond more quickly (or more slowly).

In one embodiment, VF frequency content is demonstrated by applying theWelch method of frequency estimation using 2048 data points and a 1024sample window. The signal is taken from the defibrillator paddleschannel, which is sampled at 125 hz. The filtered signal is high-passedwith a 10 pole 2 Hz Butterworth high-pass filter. A separate spectralanalysis is performed each second. The amplitude of each spectrum isnormalized to the peak value in that time slice. FIG. 16 is a contourplot, but other methods of illustrating the three-dimensional data maybe used as would be apparent to one skilled in the art. In this example,visually, the CPR artifact from the VF signal is separated because theartifact happens in narrow multiples of the CPR rate, which was 1.6 Hz,by way of an example. CPR artifact can be removed from this spectrumsimply by setting all the multiples of a certain value such as 1.6 Hz tozero. This method removes some of the VF frequency content, but leavesenough of the VF signal to allow adequate frequency estimation. Itshould be appreciated that this disclosure offers a long-sought solutionin cardiac resuscitation.

ECG in CPR Utilizing Shock History and Compensating for Expected Noise

While a correct shock decision can be properly made during CPR, there isa subset of patient signals that contains excessive noise, which mayprevent analysis. This subset can be automatically identified andexcluded from analysis during CPR but this approach also carries therisk of excluding cases in which the automatic analysis would have beensuccessful. Amplitude of CPR artifact is much greater in patients whohave not received a defibrillation shock than in those patients who havebeen shocked. The task of analyzing during CPR is easier after a shockhas been delivered than before.

Therefore, the present disclosure adjusts analysis algorithm parametersbased on whether a shock has been delivered or not. Prior to deliveringa shock, the algorithm may be biased toward pausing CPR to allow a“clean”, meaning uninfluenced by CPR artifact, analysis and evaluationif there is any doubt about analysis accuracy. After delivering a shock,the algorithm is biased in favor of minimizing pauses and accepting theanalysis results in recognition of the fact that the artifact is smallerand the analysis quality improves after shocking.

FIG. 17 illustrates one embodiment where parameters of an ECG (or otherwaveform) are adjusted and analysis module takes into account whether apatient has been previously shocked with a defibrillator. This methodimplements a shock analysis algorithm configured to analyze thepatient's rhythm during CPR chest compressions. The nature of thesignals, and consequently the performance of the method, changessubsequent to shock delivery. The shock history is therefore used toadjust parameters and to optimize performance based on whether a shockhas been delivered, as illustrated in FIG. 17.

For further illustrative purposes, one approach is a rhythm analysisalgorithm that measures parameters X and Y to make a shock decision.Examples of parameters that might be used are the ECG amplitude,frequency, median frequency, rate of zero crossings, impedance, etc.Other parameters are possible as would be obvious to one skilled in theart. To recommend shock, the value of X is to exceed threshold 1 and Yis to exceed threshold 2. However, threshold 1 and threshold 2 maychange and be different depending on whether the defibrillator haddelivered a shock to that patient or not. In the present disclosure, therevised thresholds compensate for the fact that the pre-shock ECG isexpected to contain more noise than the post-shock ECG. Alternatively,the algorithm may implement the following formula to make a shock/noshock decision:Index=A*X+B*Y+C

Here, X and Y are measured waveform parameters, and A, B, and C areconstants. A shock is recommended if Index exceeds a threshold, andno-shock is recommended, if Index is less than the threshold. In presentsubject matter, on the other hand, the threshold value for Index changesafter a shock has been delivered, or alternatively, the values of A, B,and C change.

In a further embodiment, certain parameters, which are useful post-shockare not be reliable pre-shock if the noise sensitivity of differentmeasured parameters varies. Some parameters are so sensitive to noisethat they are not useful for patients that have not been previouslyshocked. To compensate, this method will invoke different parameters forpatients who have been previously shocked than for those that Shave notbeen previously shocked.

In a further embodiment, the Index formula, as described above, istailored to allow for three recommendations: shock, no-shock, andindeterminate. Index values above an upper threshold give a shockrecommendation, values below a lower threshold give a no shockrecommendation, and values between the upper and lower thresholds givean indeterminate result. The upper and lower thresholds, accordingly,are farther apart for patients that have not been shocked, but closertogether for patients that have been shocked resulting in moreindeterminate values for unshocked patients and fewer indeterminateresults for patients who have been shocked. This approach compensatesfor the noisy unshocked ECG signals by using a larger indeterminate zonefor unshocked patients. Patients who have been previously shocked mayhave a small or nonexistent indeterminate zone, resulting in few if anyindeterminate recommendations. In one embodiment, the Shock Index is anumerical value calculated based on measured patient waveformparameters. The algorithm recommends a shock for patients with a highshock index value, no shock for patients with a low shock index value,and gives an indeterminate result for values in the middle (near zero).

FIG. 17 further illustrates an example of an overlap between the ShockIndex values for the patients that are truly shockable versus those thatare not shockable. This overlap could cause an incorrect shockrecommendation. By way of an example, consider a system with a singleshock/no shock threshold of zero. If all patients with a shock indexvalue above zero are considered shockable, then about 1% ofnon-shockable patients receive an incorrect shock recommendation.Similarly, if all patients below zero are considered non-shockable, thenabout 1% of shockable patients would receive an incorrect no-shockrecommendation. Therefore, to minimize the number of incorrect shockrecommendations, the present disclosure offers a two-threshold approach.Only those patients above the upper threshold or below the lowerthreshold receive a definite shock recommendation; patients in theindeterminate zone receive neither a shock nor a no-shockrecommendation. The thresholds for the indeterminate zone are set insuch a way as to as to minimize the number of incorrect shockrecommendations.

There is more overlap between the Shock Index values for shockable andnon-shockable patients who have not been previously shocked than thereis for patients who have been previously shocked. As there is more noiseon the ECG signal of a patient who has not been previously shocked, anunshocked patient needs a wide indeterminate zone in order to avoidincorrect shock results. In comparison, a previously shocked patientmight have a narrow or non-existent indeterminate zone. This realizationis useful for analysis-during-CPR. Definite shock/no shockrecommendation during CPR can then be made and pauses avoided. On theother hand, if for any reason, the analysis-during-CPR is unable to makea definite evaluation and recommendation, the recommendation will be topause for a “clean” analysis rather than to proceed with an incorrecttherapy. Again, more pauses in CPR are recommended for patients who havenot been previously shocked. In such cases, patients have their ECGsignal analyzed during a pause in compressions, and thus, avoid thenoise associated with CPR on an unshocked patient.

A person skilled in the art will realize that there are many ways ofadjusting an analysis, evaluation, and recommendation of the presentsystem and method, depending on whether or not a patient had beenpreviously shocked. It is possible that the algorithm used for a patientwho has not been previously shocked might be adjusted and customized toa patient depending on the patient's needs and prior patient historyincluding ECG, CPR, and shock therapy. There are many ways of adjustingand customizing the parameters, as would be appreciated by one skilledin the art.

In another embodiment, a 12-lead interpretive algorithm that makesinterpretive statements utilizes different thresholds depending on theexpected noise level. Alternatively, an algorithm measuring the VFquality uses a measure such as AMSA, the scaling exponent, the medianfrequency, or the rotational velocity is adjusted depending on whether ashock had been delivered or not.

The present subject matter is further configured to assess a patientcondition or measure a patient parameter using signals other than an ECGsignal. For example, the patient impedance waveform is used indetermining the condition of a patient. Such an impedance waveform ismeasured with an AC signal such as 20 kHz and is demodulated into realand imaginary components. The impedance waveform is then represented ashaving a magnitude and a phase. A patient assessment module utilizes thepatients ECG in conjunction with the impedance signal to evaluate andreach a conclusion. Because the ECG signal tends to be more noisy for apatient that has not been shocked, it may be beneficial to adjust thealgorithm parameters depending on whether the patient has beenpreviously shocked.

In a further embodiment, there is the noise on the impedance signal ofan unshocked patient versus a shocked patient is not higher. In otherwords, there is no reason to adjust the processing of the impedancesignal based on the shock history. An analysis module that utilizes boththe ECG signal and the impedance signal and the ECG signal processingparameters are adjusted based on the shock history but not adjust theimpedance parameters. Shock history is used as an indicator of theamount of expected noise on an ECG signal. Other ways of anticipatingthe amount of noise include a low-frequency (<1 kHz) impedancemeasurement used as an indicator of the amount of expected noise.

To-date, defibrillators measure the patient impedance at a highfrequency (10 kHz-100 kHz). A high frequency carrier signal isadvantageous because such signal helps to separate the high-frequencyimpedance carrier signal from the ECG signal, which has a relatively lowbandwidth. Also, AC signals in the range of 30 kHz-60 kHz have beenshown to be useful for predicting the high voltage defibrillation shockimpedance. On the other hand, a low-frequency impedance measurement isused when anticipating the amount of noise that might be expected on anECG signal. When measured at a low frequency, a high impedance patientis expected to have a noisier ECG signal than a low impedance patient.Thus, an impedance measurement can be used to adjust an ECG analysisalgorithm in a manner similar to the patient's shock status.

In one embodiment, an impedance measurement is made when the electrodesare applied to the patient or when the electrodes are still in thepouch. Some electrode systems include a conductive bridge between thetwo pads while they are in the pouch. This bridge allows thedefibrillator to detect the presence of the electrodes and to assesswhether the electrode gel has dried out using the impedance measurement.The bridge can take the form of a small wire between the two pads thatis easily removed before the pads are applied to the patient. Thisallows the impedance measurement to be made while the electrodes arestill in the pouch. This impedance may correlate with the amount ofnoise on the ECG signal. Thus, for an analysis algorithm one may adjustparameters based on an impedance measurement made while the electrodesare still in the pouch.

Other ways of predicting the amount of noise to be expected on apatient's ECG signal can be engaged. For example, the electrodechemistry may contribute to the amount of noise on the ECG signal. If anelectrode with a noisier chemistry is used, algorithm parameters can beadjusted accordingly. Alternatively, some algorithms may be incongruentfor use with some electrodes. In such situation an analysis moduleengages an algorithm enabled for some electrodes but disabled for otherelectrodes.

In a further embodiment, the electrode design contributes to the amountof noise. For example, a conventional 2-wire defibrillation pad isrelatively sensitive to noise and artifacts. On the other hand, theamount of noise pickup can be reduced if a reference electrode is addedto one of the pads. A reference electrode can be useful for reducing thenoise on the ECG signal. A shielded electrode will be less sensitive tosome kinds of noise than a non-shielded electrode. Also, a therapy padwith a separate, small ECG sensing pad built in can be less sensitive tonoise. An AED analysis module is capable of engaging algorithm capableof compensating for these electrode configurations to account for theamount of expected noise. If the electrode in use produces a lower noisesignal than a conventional electrode, the selected engaged matchedalgorithm analyzes with higher accuracy, requires fewer pauses for aclean analysis, or analyzes more quickly and effectively.

In one embodiment, the defibrillator, or a medical device, determinesthe type of electrode connected to the defibrillator and/or the patient.The defibrillator is capable of detecting the electrode type based on anautomatic electrode identification scheme, such as a memory chipembedded in the electrode. Alternatively, the defibrillator isconfigured for a specific electrode type prior to use. Once thedefibrillator has determined the electrode type, the system and methodthen selects and engages algorithm parameters accordingly.

In some circumstances, automated CPR machines create electrical noise,which is picked up on the ECG signal. The noise from the CPR machine caninterfere with or aid analysis. When a defibrillator is used with aparticular automated CPR machine, the present system and method, adjustsanalysis, evaluation, and recommendation base on algorithm to accountfor the expected noise. The defibrillator, for example, becomes aware ofthe presence of a particular automated CPR machine either by signalanalysis (for example, analysis of the impedance signal), by operatorselection, by direct communication between the two machines, or by othermechanisms.

In a further embodiment, the electrode pad placement also contributes tothe amount of noise observed on an ECG signal. While an anterior-lateral(A-L) position is most common, an anterior-posterior (A-P) placement issometimes used. The A-P placement can result in a noisier ECG signalbecause the rescuer's hands are placed directly on top of the anteriorpad. As such, the analysis module engages algorithm based on padplacement. The defibrillator may determine that the pads are likely tobe placed in A-P position based on the pad type. For example, aninfant-child electrode is more likely to be used in the A-P positionbecause there is a limited amount of room on the chest of an infant, soit is reasonable to expect an infant-child electrode to be noisier thanan adult electrode. Alternatively, an operator makes an AED selectionand instructs the defibrillator to use the algorithm tailored to A-Pposition.

AED Operation Dependent on Previous Analysis Results

To date, defibrillators prompt for CPR for a fixed amount of time eachCPR period, which is typically two minutes. While this fixed time periodmay be programmable at the time the AED is originally set up, itgenerally does not vary from one CPR cycle to another. A defibrillatormay prompt the user to stop CPR at the end of the CPR period so that arhythm analysis may be performed. The rhythm analysis is then generallyperformed once every CPR period. Thus, a given patient may have manyrhythm analyses performed during the course of their care. The algorithmused for each of these rhythm analyses generally does not change fromone analysis to another. In general, to-date, algorithms may measuresome signal characteristics and compare those measured characteristicsto thresholds to make a shock/no-shock decision. Typically, the samecharacteristics are measured and the same thresholds applied each timethe rhythm is analyzed.

In contrast to prior attempts in the field, which as described abovegenerally exhibit the same behavior each CPR period regardless of theresults of previous rhythm analysis results, in the present disclosure,a defibrillator behavior in a given CPR period is dependent on previousrhythm analysis results. One aspect of this embodiment varies the CPRperiod and the associated prompts as a function of previous rhythmanalysis results. In this embodiment, if the first AED analysis yields a“no shock advised,” the CPR period can be different than if the firstanalysis result is “shock advised.” In one embodiment, after an initial“no shock advised,” the CPR period is modified such that CPR isperformed indefinitely. No further rhythm analyses is then performed onthat patient. Alternatively, the CPR period is lengthened. For example,the CPR period is lengthened from two minutes to five minutes.Alternatively, the AED is set up to prompt for CPR until another eventis detected. Such an event can be a user action, such as a button press,or can be a device-detected event, such as electrode disconnection.

On the other hand, if the first rhythm analysis result is “shockadvised,” the device operates with normal CPR periods (i.e. an analysisevery 2 minutes). These CPR periods can continue as long as the patientis treated by the defibrillator or they could be altered when anotherevent is detected. Such an event might be, by way of an example, a useraction, such as a button press, or it could be a device-detected event,such as electrode disconnection.

In a further embodiment the defibrillator changes the duration of theCPR period during the course of patient treatment. For example, the AEDmay be set up for an initial CPR period before the first analysis. Byway of an example, this CPR period is programmable with a durationbetween 15 seconds and 3 minutes. If the first rhythm analysis yields a“no-shock” result, then the CPR period could change to another longervalue, possibly 5 minutes. If a subsequent rhythm analysis yields a“shock advised” result, the CPR period could then change to a shorterperiod, possibly 2 minutes. Such a defibrillator has three different CPRperiods, one for initial CPR, one for application after a “no shock”analysis, and one for application after a “shock advised” analysis. Oneskilled in the art will realize that other CPR periods could be appliedunder other circumstances that the AED may encounter.

In still another embodiment, a defibrillator alters the analysisalgorithm as a result of previous analysis results. For example, the AEDmay use one analysis algorithm for the initial analysis and a differentalgorithm for subsequent analyses. If the initial analysis yields a “noshock” advised decision, then the subsequent analyses are performed witha different algorithm than if the initial analysis were “shock advised.”Such an AED's analysis module has three algorithms, an “InitialAnalysis” algorithm, a “Previously Non-Shockable” algorithm, and a“Previously Shockable” algorithm. The appropriate analysis algorithm ischosen based on previous analysis results. The algorithm can be chosenbased on the results of the first analysis done on a patient, on theanalysis immediately prior to the given analysis, or on other analysesthat the device has performed on the patient. In one scenario thedefibrillator uses the “Previously Non-Shockable” algorithm until ashock or a shockable rhythm has been observed, and then the “PreviouslyShockable” algorithm is engaged from then on. Similarly, other analysisalgorithms are defined for use under other scenarios and applied undervarious circumstances as would be appreciated by one skilled in the art.

In a further embodiment, a medical device or a defibrillator alters theanalysis module's algorithm based on a patient analysis other than ashock/no-shock decision. For example, if a patient has an initiallynon-shockable rhythm that contains QRS complexes, the algorithm used forsubsequent analyses can be different than for an initially asystolicpatient. Both rhythms are non-shockable, but different algorithms can beused for subsequent analyses. Similarly, different algorithms can beinvoked for patients with initial bradycardia, normal sinus rhythm,supra-ventricular tachycardia, or other non-shockable rhythms. Along thesame lines, different algorithms can be employed for subsequent analysesfor a patient with initial “course” ventricular fibrillation (VF) asopposed to “fine” VF. Course VF can be distinguished from fine VF basedon the peak-peak signal amplitude, or can be based a frequency analysisof the VF signal, an amplitude-frequency analysis such as AMSA, or otherVF analysis method. The analysis algorithm can also be chosen based onprevious patient hemodynamic information. A subsequent analysis in apatient that had previously exhibited a pulse can be different from apatient who never had a pulse detected. The algorithm selection can bebased on the patient analysis at the time the medical device, such asand AED, is initially applied, on a patient analysis immediately priorto a given analysis, or an analysis performed at another point in time.For these purposes, the patient analysis can be an ECG analysis, or itcan be an analysis of another patient signal or combination of signals.An analysis module includes an algorithm for quantitative evaluation ofpatient data and leads to a decision about a patient condition.

In a yet further embodiment, the analysis module can select from aplurality of algorithms and can further provide for flexible switchingbetween algorithms. In one example, an algorithm is selected from aplurality of different algorithms for an initial analysis and forsubsequent analyses, a different algorithm is selected. Further, theanalysis module comprises different algorithms capable of analyzing thesame parameters, but could apply different coefficients or differentthresholds to each parameter. Alternatively, different parameters couldbe analyzed or a different process could be used for the differentalgorithms. A system of checks and balances can also be applied whereseveral algorithms process parameters to verify results.

One way to incorporate multiple factors into a shock decision isachieved through the use of a formula such as:

-   -   ShockIndex=A*VFScore+InitialShockable+B*SubsequentVF+C, where        ShockIndex=An overall numerical rating of likelihood the patient        needs a shock    -   VF Score=A numerical rating of how much like VF the current        rhythm is InitialShockable=A variable that takes on three        distinct values depending on whether the initial rhythm was        shockable, non-shockable, or unknown. The values would be        calculated by regression analysis.    -   SubsequentVF=0 if no VF has been observed after the initial        analysis, 1 if subsequent VF has been observed.    -   A, B, and C=Numerical weighting factors calculated by regression        analysis.

As an example, a case in which a patient starts off in a non-shockablerhythm and fibrillates after 7 minutes of treatment and the AED isconfigured to prompt for initial CPR is considered below. Here is thetime course of the device operation:

Time Device Behavior Power On: Device prompts for CPR  2 Minutes:Initial rhythm analysis is performed. If SHOCK ADVISED, analysiscontinues every 2 minutes. If NO SHOCK ADVISED, analysis intervalchanges to 4 minutes.  6 Minutes: Rhythm analysis in “InitiallyNon-Shockable” mode. This mode is biased toward higher specificitybecause of the initial non-shockable rhythm. If SHOCK ADVISED, analysisinterval would switch back to 2 minutes. If NO SHOCK ADVISED, analysisinterval stays at 4 minutes. 10 Minutes: Rhythm analysis in InitiallyNon-Shockable mode with 2 previous no-shock results. This mode is biasedtoward even higher specificity. If SHOCK ADVISED, analysis intervalswitches back to 2 minutes (as below). 12 Minutes: Rhythm analysis inInitially Non-Shockable mode with subsequent VF.

As would be appreciated by one skilled in the art, this approach can beapplied to an AED configured to analyze during CPR. Both the rhythmanalysis algorithm and the analysis intervals can be influenced byhistory as described above.

In a further embodiment, analysis-during-CPR algorithms include a“continuous mode” which analyzes continuously during CPR and interruptsthe CPR period if VF is detected. A continuous mode is recommended forpatients with a high likelihood refibrillating, and not recommended if apatient has a very low probability of going into VF. If the odds of VFare low, a continuous mode can increase the likelihood of incorrectlyindicating “shock advised” while providing little chance of detectingand terminating VF sooner. In such cases, rhythm analysis may carry arisk of an incorrect result. Thus, over-analyzing a patient unlikely tobe in VF may do more harm than good.

To mitigate this concern, activation of continuous mode is madecontingent on the patient history. An AED treating a patient with aninitially non-shockable rhythm performs a rhythm analysis during CPR atthe regular intervals (e.g. 2 minutes). However, an AED treating apatient with an initially shockable rhythm can switch to continuous modebecause refibrillation is likely and the patient will benefit fromearlier VF termination.

One skilled in the art will realize that there are many ways ofdisplaying an analog value. The examples shown here are graphical, butit is possible that numbers could be displayed as well, or combinationsof numbers and graphics. The Shock Index value could be displayed, orthe probability of cardiac rhythm, VF, could be displayed, or some othernumerical indicator that relates to the waveform.

Real-Time Cardiac Rhythm Quality & Rhythm Assessment Meter

The “VF Quality” degrades over time if CPR is not provided. VF qualitycan be measured in one or more ways, however, to-date, measuring VFquality during CPR has posed a significant challenge. Several methodshave been proposed for using VF quality to guide therapy. One methodapplies shock immediately if VF quality is good and provides two minutesof CPR if it is poor. Another approach is to provide CPR until VFquality reaches a predetermined level.

The present embodiment offers a real-time VF quality indication to arescuer/user during CPR. The indication might be a comparative scale ofsome type, such as a continuum scale, a gauge or a bar graph, a trendline, a pie chart, or a colorimetric scale, a digital scale, etc. Theindication may also be tactile or auditory. The visual indicationillustrates the quality of a VF in real time and allows the user toquickly assess the status and trajectory of the cardiac rhythm,allowing, for example, the CPR to continue as long as the quality of thecardiac rhythm, such as VF, is improving. One skilled in the art willrealize that there are many ways of displaying an analog value. Theexamples shown here are graphical, but it is possible that numbers couldbe displayed as well, or combinations of numbers and graphics. The ShockIndex value could be displayed, or the probability of VF could bedisplayed, or some other numerical indicator that relates to thewaveform. This approach is superior to doing CPR for a fixed period oruntil a fixed level of quality disregarding the actual individualreal-time patient data and status. If, for example, the VF quality isobserved as not improving, other interventions are engaged rather than acontinued CPR.

If VF quality continues to improve, the rescuer continues CPR with ahigher degree of confidence and without unnecessary stopping or pausingat a predetermined threshold as may be recommended by existingguidelines. If VF quality is not improving it is possible that CPRquality is poor and that the caregiver should change the depth, rate orother parameter. It is possible that optimal CPR depth and rate may varyfrom one individual to another. This approach allows CPR to be adjusteddynamically based on the results with a given patient. VF qualityindications given to the user allow assessment of the current VF qualityand of whether it is improving.

Further, FIGS. 18A-C illustrate embodiments of representations of shockrecommendation using a logistic regression. The inputs for logicalregression comprise numerical measurements of the ECG and impedancesignals. The output of the logistic regression approach is a number suchas a shock index number as illustrated in FIG. 18A. A positive shockindex value indicates a shockable rhythm, a negative shock indexindicates a non-shockable rhythm, and a value in the middle isindeterminate. A shock index value of zero means there is a 50-50 chancethat the patient has VF. By way of an example, for “accuracy emphasis”mode all values between −2.5 and +2.5 are considered indeterminate;shockable rhythms are >2.5, non-shockable rhythms are <−2.5. Such analogmeter, as illustrated in FIGS. 18B and 18C, exemplifies a “RhythmAssessment Meter” and is, for example, displayed on a manualdefibrillator screen. The far left of the meter scale is −5, the farright is +5, and the indeterminate zone goes from −2.5 to +2.5.

FIG. 19 is an example of waveform assessment trend line. Here, theprobability of VF in a patient starts at time T_(0:00) with anon-shockable rhythm, and then transitions to a shockable rhythm afterapproximately 43 seconds of CPR. The trend line illustrates theprobability of VF (as shown in FIG. 19), the shock index, or anothernumerical value relating to the patient waveform. The trend lineillustrates the regions that should be considered shockable,non-shockable, and indeterminate, as presented in FIG. 17. The trendline illustrates when an individual is refibrillated and how long he/shehad been in fibrillation. Also, if the trend line is steadily in theshockable or non-shockable zone, then the operator's confidenceincreases as to correctness of the rhythm analysis, whereas a numberthat is not consistently displaying is an indicator of uncertainty. Theshock index can be calculated continuously, continually or at discreteintervals. The intervals could be based on the CPR interval, on theartifact level on the signal, the Central Processing Unit (CPU) burdenfor calculation, and/or other parameters. There may also be intervalsduring which no data is available.

When analyzing a noisy signal, like an ECG during CPR, it is not alwayspossible to make a definitive interpretation. Providing an analog scaleto the user conveys not only the recommendation, but the confidencelevel that is associated with that recommendation. A value far into atreatment zone, either shockable or non-shockable, is associated with afar greater confidence than a value near a boundary. Part of the valueof the analog scale is that some users may make treatment decisions at adifferent confidence level than other users. If the artifact stillcannot be filtered and/or the artifact levels are somehow excessive,persistent, or the signal contains noise, automatic analysis mayindicate a different course of action and shift to a backup process orsequence.

In one embodiment, “blind” shocks are given to pulseless patientswithout assessing the rhythm. The understanding in the field is that theharm to the patient from an unnecessary shock is low in comparison tothe harm to the patient from withholding a necessary shock. Rescuers,therefore, are disinclined to stop CPR to assess the rhythm, and chooseto proceed with “blind” shocks. The present disclosure enables rescuersto monitor, in real time, what is the next most advantageous step he/sheshould take for the optimal benefit of the patient within certainconfidence level.

A large sudden change on the rhythm assessment meter may indicate theonset of VF, while a small change may simply be the result of remaining,unfiltered noise. The device is further capable of voice prompts,flashing lights, signals, etc. when a certain zone with a certainconfidence level is reached. The meter can further facilitate answerswhen an operator does not trust a filtered ECG signal. The meter is mostvaluable when an operator/rescuer has no way of knowing whether to trustthe filtered ECG signal and when resuscitation needs to stop CPR toobtain a clean signal.

By way of an example, when the meter is in the indeterminate zone, thewaveform may not be trusted. If the meter is in the shockable zone, thefiltered waveform display may be helpful by providing an indication ofthe VF amplitude, a feature that provides an indication of the health ofthe patient. If the meter is in the non-shockable zone the rescuer mayfind it useful to know whether the patient is in asystole or pulselesselectrical activity (PEA). If the patient appears to have regular,normal-rate QRS complexes the rescuer may choose to stop CPR to checkfor a pulse. Conversely, if they are confident that no QRS complexes arepresent they may choose to skip their normal pulse check.

Integration of Cardiac Rhythm Analysis During CPR into a DefibrillatorAlgorithm

In a further embodiment, a defibrillator, such as an AED, integrates aCPR prompting sequence. FIGS. 20, 21 and 22 illustrate steps forflexibly incorporating CPR prompting sequence into another device suchas a defibrillator. In one embodiment, an AED user or a remote device isprompted to setup or flexibly adjust with setup options. Setup optionscan be decided upon, by way of an example, by a Medical Director of theperson operating the defibrillator to conform to the treatment protocolhe or she orders for all persons operating under his or herdirectorship. As such, the device can have provision to store theelected setup options so that the device prompts according to theordered treatment protocol and/or algorithm. Memory storage may be madeby nonvolatile memory, flash memory, disk memory or similar device, inother devices and communicated to the device by a wired or wirelesscommunication channel, including the Internet. Other options arepossible.

FIGS. 20, 21 and 22 detail the three general options, termed hereinPeriodic Mode, Continuous Mode and Minimum CPR Time, starting withprompts in 2001 2101 or 2201, respectively, according to the choice madeby the Medical Director, for example. The prompting may start after thedevice is turned on, or following additionally an analysis without CPRor following additionally an Initial CPR period.

FIG. 20 is a flowchart illustrating the method 2000 for prompting andinteracting with the analysis algorithm if Periodic Mode has beenchosen. According to operation 2001 the user is prompted to perform CPR.According to an operation at 2002 the rhythm analysis begins, silentlyin the preferred embodiment. This begins toward the end of the CPRperiod. The amount of time before the end of the CPR period to begin therhythm analysis is determined by the time it will take variousoperations to take place such that the device is ready to shock at theend of the CPR period without pause. This will be determined by suchfactors as the amount of data necessary for rhythm analysis and the timenecessary to acquire that data, the computation time of the algorithm,and the time necessary to charge the defibrillator.

According to a decision step at 2003 the result of the analysisalgorithm determines the operations subsequently taken.

If the analysis algorithm determination is shockable, according to anoperation at 2004 the defibrillator is charged.

According to a next operation at 2005 the device waits until the end ofthe CPR period. This might be necessary, for example, if the time ittakes to perform various operations before operation 2005 is variable,and the longest possible must be taken into account in operation 2002.Alternatively, the device could prompt for shock delivery as soon as thecharging is complete.

Optionally, according to an operation at 2006, the user is prompted tostop CPR. This might not be necessary if it is possible to safely shockwhile doing CPR, as would be the case with a mechanical CPR device orwhen the rescuer performing CPR is wearing gloves or other barrier toprevent being shocked.

According to a next operation at 2007, the shock is delivered. It may bedelivered with or without pressing a shock switch.

If the analysis algorithm determination is indeterminate, according toan operation at 2008, the device waits until the end of the CPR period.

According to an operation at 2009, the user is then prompted to stopCPR.

According to an operation at 2010, the device then performs a rhythmanalysis using an algorithm which is appropriate for patients who arenot receiving CPR, as is common in the state of the art.

If the analysis algorithm determination is nonshockable, according to anoperation at 2011, the device waits until the end of the CPR period.

Optionally, according to an operation at 2012, the algorithm indicatesto the user that no shock is advised.

No matter the analysis algorithm result at an operation 2003, processingproceeds to the operation at 2001.

FIG. 21 is a flowchart 2100 for illustrating the method of prompting andinteracting with the analysis algorithm if Continuous Mode has beenchosen. According to operation 2101 the user is prompted to perform CPRand the rhythm analysis starts at the same time. The rhythm analysis issilent in the preferred embodiment.

According to a decision step at 2102 the flowchart the result of theanalysis algorithm determines the operations subsequently taken. The CPRperiod is nearly over if the amount of time before the end of the CPRperiod is such that the device will be ready to shock at the end of theCPR period without pause. This will be determined by such factors as theamount of data necessary for rhythm analysis and the time necessary toacquire that data, the computation time of the algorithm and the timenecessary to charge the defibrillator.

According to a decision step at 2103 the result of the analysisalgorithm determines the operations subsequently taken.

If the result of the analysis algorithm is no or indeterminate, thedevice continues the CPR period and continuous to perform the rhythmanalysis according to decision step 2101.

If the analysis algorithm determination is shockable, according to anoperation at 2104 the defibrillator is charged.

Optionally, according to an operation at 2105, the user is prompted tostop CPR. This might not be necessary if it is possible to safety shockwhile doing CPR, as would be the case with a mechanical CPR device orwhen the rescuer performing CPR is wearing gloves or other barrier toprevent being shocked.

According to a next operation at 2106, the shock is delivered. It may bedelivered with or without pressing a shock switch. Unlike Periodic Mode,this occurs as soon as the rhythm is determined to be shockable nomatter how much of the CPR period has completed.

If the decision step at 2102 is that the CPR period is nearly over, thenaccording a decision step at 2107, the subsequent operation steps aredetermined by the analysis algorithm result, but in a different fashionthan is outlined in decision step 2103. It is possible that thealgorithm works slightly or completely differently in step 2107 than in2103. For example, there may be an advantage to a different tradeoffbetween sensitivity and specificity in the algorithm because the impactto the patient of an incorrect nonshockable result in processing step2103 is for the user to perform more CPR, but in processing step 2107 itwould result in failure to deliver therapy to a patient who needed it.

If the analysis algorithm determination in step 2107 is indeterminate,according to an operation at 2108, the user is then prompted to stopCPR.

According to an operation at 2009, the device then performs a rhythmanalysis using an algorithm which is appropriate for patients who arenot receiving CPR, as is common in the state of the art.

If the analysis algorithm determination is nonshockable, optionally,according to an operation at 2012, the algorithm indicates to the userthat no shock is advised.

No matter the analysis algorithm result at an operation 2103 or 2107,processing proceeds to the operation at 2101.

FIG. 22 is a flowchart 2200 for illustrating the method of prompting andinteracting with the analysis algorithm if Minimum CPR Time has beenchosen. This option is like Continuous Mode as illustrated in FIG. 21,but ensures that the patient receives a minimum amount of CPR.

According to an operation 2201 the user is prompted to perform CPR.

According to a decision step at 2202 the result of the analysisalgorithm determines the operations subsequently taken. The step teststo see if a minimum amount of CPR has been given. The amount of CPR canbe determined by either the duration of one CPR period or by separateinput from the Medical Director or by some equivalent means.

If the minimum amount of CPR has not been given, according to anoperation at 2203 operation proceeds as outlined in operation 2001 inflowchart 2000. If the minimum amount of CPR has been given, accordingto an operation at 2204 operation proceeds as outlined in operation 2101in flowchart 2100.

In a further embodiment the system and method for electrocardiogramanalysis for optimization of chest compressions and therapy and deliveryinclude the rhythm assessment meter device and the filtered waveformdisplay where the meter and the display complement and corroborateresults of one another and the system arms a rescuer/operator withresults based on a certain confidence level. For example, the waveformchanges its appearance when the rhythm assessment meter is in theindeterminate zone. In another example, if the waveform can not betrusted, the waveform changes to a specific color, gray—for example, orperhaps to a dashed line. Alternatively, a visual or audible indicationis given when the filtered waveform provides low confidence level to therescuer.

In FIG. 23, the signal 2301 represents the ECG signal taken from acardiac arrest patient who was receiving mechanical chest compressionswith a pause. The signal 2303 is same signal that has been filtered withat least one filter mechanism (not shown) comprising a comb filter, suchas an embodiment of the filter mechanism 425 described above. In thisembodiment, at least one filter mechanism is implemented using an IIRfilter, which incorporates a “memory” that takes time to respond tochanges in the ECG signal 2301.

During a compressions period 2302, the start of the ECG signal 2301shows a significant amount of compression artifact that would interferewith rhythm interpretation. The at least one filter mechanism hasremoved compression artifact in a portion of the ECG signal 2301 duringthe compressions period 2302. This filtering revealed that the patientis experiencing VF (ventricular fibrillation), as shown in a portion ofthe filtered signal 2303 during the compressions period 2302. As can beseen, that portion of the ECG signal 2301 is noisy and that portion ofthe filtered signal 2303 is relatively clean during the compressionsperiod 2302.

During a pause period 2311 (i.e., when compressions are paused) thesituation is reversed—a portion of the ECG signal 2301 is relativelyclean and a portion of the filtered signal 2303 contains artifact. Dueto the nature of the IIR filter used in this embodiment of at least onefilter mechanism, the amount of artifact on that portion of the filteredsignal 2303 diminishes over time during the pause period 2311. Thisdelay in reducing the artifact can be problematic for EMS rescuers whomay be used to rapidly assessing the patient's rhythm during acompression pause. This behavior of the at least one filter mechanism(i.e., injecting artifact during a compression pause) is inherent in acomb filter structure and may also occur with other filters such asnotch filters and adaptive filters.

After compressions restart in a compressions period 2321, there isartifact on both the ECG signal 2301 and the filtered signal 2303. Overtime the at least one filter mechanism “re-learns” the shape of theartifact, but the response delay can be undesirable. Pause periods suchas the pause period 2311 can occur when the rescuer stops and restartsthe compression device. Pause periods may also happen periodically ifthe device is programmed to include automatic ventilation pauses in apattern such as a 30:2 compression/ventilation ratio. The artifact thatwould be inserted during a compression pause could be annoying torescuers because they may be looking at the ECG during a pause. In thissituation the at least one filter mechanism may not perform as theywould likely expect—the at least one filter mechanism removes theartifact during the compressions and inserts it during an early portionof a CPR pause.

In FIG. 24, the signal 2401 represents the ECG signal taken from thesame cardiac arrest patient who was receiving mechanical chestcompressions at a different time period in which a defibrillation shockwas delivered to the patient. The signal 2403 is same signal that hasbeen filtered with the at least one filter mechanism (not shown). Duringa non-shock period 2411, a portion of the ECG signal 2401 showscompression artifact and a portion of the filtered signal 2403 shows theECG signal with the artifact removed.

During a time period 2421, a defibrillation shock is delivered to thepatient. No ECG data is available during the shock, so a portion of theECG signal 2401 is represented as a flat line in the period 2421. Duringthe time period 2421, a portion of the filtered signal 2403 has somecompression artifact because the at least one filter mechanism must“re-learn” the artifact.

In a post shock time period 2431, eventually the at least one filtermechanism learns the artifact and begins to show a cleaned-up signallater in the time period 2431, but the delay can be undesirable in someapplications. Filter delays such as this are particularly undesirableafter a defibrillation shock because they interfere with the ability ofrescuers to determine whether the shock succeeded and deciding whetheranother shock may be necessary.

The examples shown above use a comb filter for the purposes ofillustration, similar problems are likely to be observed with any IIRfilter or FIR filter with a long memory. Similarly, adaptive filters maytake time to respond to signal changes. In general, any filteringtechnique that has a “learning” aspect may experience similar problems.

Further, there may be other types of disturbances that can cause similarartifacts as the at least one filter mechanism relearns the artifact.Various embodiments described below can be used to detect thesedisturbances.

In one embodiment, a monitoring device (e.g., a monitor) is aware ofevents such as a monitoring lead falling off or a change in themonitoring lead vector. This information is communicated to themonitoring device so that appropriate compensation can be provided aswill be described below. For example, in some embodiments, theinformation is provided to the at least one filter mechanism so that itScan make the appropriate compensation. If the monitor is an externaldefibrillator, then the at least one filter mechanism can also beconfigured to compensate for shock disturbances.

In other embodiments, pauses in compressions can be detected by themonitoring device using signal analysis of the ECG signal. For example,mechanical compressions have a very specific frequency signature. Theabsence of that signature could be taken as evidence that thecompressions have stopped. Another method of detecting mechanicalcompressions is to run an inverse comb filter and detect the amplitudeof the resulting signal. Both of these techniques may take time toacquire enough data and may be appropriate in less time urgentapplications.

In other embodiments, pauses in compressions can be detected by themonitoring device using the impedance signal if it is available. Onemethod of detecting the presence of compressions is to take the RMSvalue of the impedance signal over a 600 ms period. If the RMS value isabove a threshold then it is likely that compressions are occurring.This method may have less delay than ECG signal analysis, but also maybe less specific. Other types of motion may affect the RMS value,causing erroneous detection of compressions.

In still other embodiments, the CPR machine can communicate with themonitoring device when compressions are going to be started or stopped.The monitoring device can then compensate for the disturbance. In oneparticular embodiment, the CPR machine would provide advance notice thatcompressions are going to be started or stopped so that the monitoringdevice can compensate before the memory of the at least one filteringmechanism is disturbed.

In yet another embodiment, the monitoring device may be able toanticipate some signal disturbances simply by knowing what kind of CPRmachine that is being used. For example, some CPR machines may provideprecisely-controlled timing for a 30:2 compression/ventilation ratio.The monitoring device may then anticipate the ventilation pause simplybased on the elapsed time since the last known pause (e.g., detectingusing the ECG or impedance signal). This method is particularlyadvantageous because it requires no communication between the CPRmachine and the monitoring device but still allows the pause locationsto be precisely identified.

In response to detecting a disturbance (e.g., via any of the variousembodiments described above), the monitoring device is configured toselectively compensate for the detected disturbance. Various embodimentsdescribed below can be used to provide the compensation.

In one embodiment, once a pause in compressions is identified themonitoring device can choose to automatically switch from displaying thefiltered signal to displaying the unfiltered signal. This would avoidthe problem seen in FIG. 23 in which the filtered signal has moreartifact than the original ECG signal during a compression pause.However, when compressions are restarted the at least one filteringmechanism still needs to re-learn the artifact.

In another embodiment, after a disturbance it is possible to make the atleast one filtering mechanism learn more quickly by temporarily reducingthe quality factor (i.e. the “Q”) of the at least one filteringmechanism. Note, quality factor of the at least one filtering mechanismis described above in conjunction with FIG. 7. In one embodiment, the atleast one filtering mechanism implemented using a comb filter has arelatively high Q value of 16. The Q of the comb filter can be changedrelatively easily by changing the value of “b” in the comb filtertransfer function. For example, the Q could be reduced to a value of 2for one second after a disturbance, and then switched back to 16. Thiswould allow the filter to recover relatively quickly when compressionsare started, a monitoring lead is attached, or after a shock has beendelivered.

In still another embodiment, the disturbance to the filtered ECG signalis reduced by controlling the CPR machine to restart the compressionsafter a pause in a manner in which the new compression group is in-phasewith the previous compression group. For example, if the compressionrate is 100 compressions per minute, each compression requires 0.6seconds. If compression pauses are arranged to be an integer number of0.6 second intervals, then the new compression group will be in-phasewith the previous compression group. This reduces the amount ofre-learning that the at least one filtering mechanism needs to do.

In yet another embodiment, if the CPR machine restarts compressionsin-phase with the previous compression group and the monitoring deviceknows where the compression pause starts and ends, then it is possiblefor the monitor device to “protect” the at least one filtering mechanismduring the pause. This would allow the at least filtering mechanism toretain its memory of the artifact during the pause rather than“unlearning.” Then, when compressions are restarted the at least onefiltering mechanism could be re-engaged without having to relearn theartifact again. This would avoid a significant disturbance of the filterfor every compression pause.

One method of protecting the at least one filter mechanism during apause is simply to stop the at least one filtering mechanism (i.e.prevent it from accepting any new inputs, prevent it from shifting thedelay lines, and prevent it from calculating any new outputs). Then theat least one filtering mechanism is restarted at the appropriate timewhen compressions are resumed. The at least one filtering mechanism isrestarted at substantially the same point in the compression cycle as itstopped. As described above, this could be done if there wascommunication between the CPR machine and the monitor device, or if themonitoring device had advance knowledge of the length of the compressionpause.

FIG. 25 shows one embodiment of a filter mechanism 2500 with aprotection switch 2502. The switch 2502 allows the filter mechanism 2500to operate normally during chest compressions but to transition to aprotected mode during a pause. In the protected mode the input to thenumerator delay line is taken from the output of the numerator delayline is rather than the input signal. This recirculates the existingdata (which contains compression artifact) through the filter mechanism2500 during a pause, which prevents the unlearning effect. Whencompressions are restarted the input to the numerator delay line isswitched back to the input ECG and the filter mechanism 2500 beginsremoving the CPR artifact without having to relearn the artifact shape.This method has the advantage that the filter mechanism 2500 could berestarted at any point in the compression cycle and the filter wouldstill be in sync with the artifact.

In another embodiment, the at least one filter mechanism is protectedduring a compression pause by changing the Q of the filter. A filterwith a high Q learns the new signal very slowly. If the Q of the filterwas made extremely high during a compression pause it is possible thatlittle or no new learning would happen during the pause. If the Q wasswitched back to the normal value when compressions are restarted thefilter would still be in sync with the artifact and the need forlearning would be avoided. A similar technique could be used with anadaptive filter that avoids relearning during compression pauses.

In this description, numerous details have been set forth in order toprovide a thorough understanding. In other instances, well-knownfeatures have not been described in detail in order to not obscureunnecessarily the description. A person skilled in the art will be ableto practice the present invention in view of this description, which isto be taken as a whole. The specific embodiments as disclosed andillustrated herein are not to be considered in a limiting sense. Indeed,it should be readily apparent to those skilled in the art that what isdescribed herein may be modified in numerous ways. For instance, themechanical chest compression devices described above may operate atdifferent frequencies than those described above, have differenttolerance thresholds than those described above, have differentharmonics than those described above, or any combination thereof.Indeed, the frequencies, tolerances, harmonics, and any other variablesor values pertinent to the disclosed technology that are discussed orotherwise presented herein are provided only as certain examples.Modifications to the disclosed technology can include equivalents towhat is described herein. In addition, the invention may be practiced incombination with other systems.

The following claims define certain combinations and subcombinations ofelements, features, steps, and/or functions, which are regarded as noveland non-obvious. Additional claims for other combinations andsubcombinations may be presented in this or a related document.

What is claimed is:
 1. A system for treating a cardiac arrest patient,the system comprising: an ECG monitor; and a mechanical chestcompression device wherein, the mechanical chest compression device isconfigured to automatically deliver a series of chest compressions andventilation pauses, the ECG monitor is configured to filter the ECGsignal to remove the compression artifact and produce a filtered ECGsignal, and the ECG monitor is configured to display the filtered signalwhen the mechanical chest compression device is delivering chestcompressions and displays the unfiltered ECG signal during ventilationpauses, wherein the ECG monitor includes a filter configured to performthe filtering of the ECG signal by attenuating a plurality of regularlyspaced bands, and wherein the ECG monitor is configured to change aquality factor (Q) of the filter responsive to an event.
 2. The systemof claim 1, wherein the ECG monitor is configured to identify theventilation pauses.
 3. The system of claim 2, wherein the ECG monitor isfurther configured to automatically switch from displaying the filteredECG signal to displaying the unfiltered ECG signal responsive toidentifying the ventilation pauses.
 4. The system of claim 1, whereinthe mechanical chest compression device is configured to transmit asignal to the ECG monitor indicative of the ventilation pauses.
 5. Thesystem of claim 1, wherein the filter comprises a comb filter.
 6. Thesystem of claim 1, wherein the event includes an attachment of amonitoring lead to the ECG monitor, a delivery of a shock, or abeginning of the delivery of chest compressions.
 7. The system of claim1, wherein the ECG monitor is configured to temporarily reduce the Q ofthe filter from a first value to a second value responsive to the event.8. The system of claim 7, wherein the ECG monitor is further configuredto restore the Q of the filter from the second value back to the firstvalue after a period of time.
 9. The system of claim 7, wherein theperiod of time is one second.
 10. The system of claim 7, wherein thefirst value is
 16. 11. The system of claim 7, wherein the second valueis
 2. 12. A system for treating a cardiac arrest patient, the systemcomprising: an ECG monitor; and a mechanical chest compression devicewherein, the mechanical chest compression device is configured toautomatically deliver a series of chest compressions and ventilationpauses, the ECG monitor is configured to filter the ECG signal to removethe compression artifact and produce a filtered ECG signal, and the ECGmonitor is configured to display the filtered signal when the mechanicalchest compression device is delivering chest compressions and displaysthe unfiltered ECG signal during ventilation pauses, wherein the ECGmonitor includes a filter configured to perform the filtering of the ECGsignal by attenuating a plurality of regularly spaced bands, and whereinthe ECG monitor includes a filter protection switch configured to allowthe filter to operate normally during the chest compressions.
 13. Thesystem of claim 12, wherein an input to a numerator delay line of thefilter is the ECG signal.
 14. The system of claim 12, wherein the filterprotection switch is further configured to switch to a protected modeduring the ventilation pauses.
 15. The system of claim 14, wherein aninput to a numerator delay line of the filter is taken from an output ofthe numerator delay line when the filter protection switch is in theprotected mode.
 16. The system of claim 15, wherein the filterprotection switch is further configured to switch back from theprotected mode to allow the filter to operate normally again responsiveto a resuming of the chest compressions after the ventilation pauses.17. The system of claim 16, wherein the input to the numerator delayline of the filter is the ECG signal when the filter protection switchis no longer in the protected mode.
 18. The system of claim 12, whereinthe filter comprises a comb filter.
 19. The system of claim 12, whereinthe ECG monitor is configured to identify the ventilation pauses. 20.The system of claim 19, wherein the ECG monitor is further configured toautomatically switch from displaying the filtered ECG signal todisplaying the unfiltered ECG signal responsive to identifying theventilation pauses.
 21. The system of claim 12, wherein the mechanicalchest compression device is configured to transmit a signal to the ECGmonitor indicative of the ventilation pauses.